My son has the same behavior. Rewards are not always a good idea because then a reward will be expected or else. I have times when my son will demand a reward and there isn't one available. All I can say is that you gotta make it clear to him immediatly that the behavior is not acceptable. If you try to take him to his room then the fight becomes about if you can take him to your room, he will forget about the slapping your face. It has to be instant, and unpleasent. If you show him that you can be maniputated then that is what he will do. This is my experience not only raising my son, but it's what was done to me when I was a child. I would use anything to manipulate my punishment. IMO. I'm no professional, just experienced.

Scyllas... the master runaway kid catcher.

As others mentioned, not knowing the child makes suggestions hard.
My 3ft + tall, 3 year old can already to considerable damge. we had an episode today and I am even now nursing my wounds and resting as he finally sleeps.
My son I had transfer biting into a hard aggressive kiss. But he will still kick me or head butt me at what seems strange and unexpected moments and catches me offguard still. We are spending a week at a different home than usual, our home away from home, and he is usually difficult the first two days.
No matter the cause, you grandson is definitely what the commmunity calls "stimmed". He is overexcited. And likely overwrought.
I was a child with many OCD behaviors and Asperger's runs in our family.
I know for me, break in schedule could cause me to meltdown, I could express my displeasure verbally though. Also I could not stand for my belongings to be moved even one hair away from the measured distance I had them from the door. I could tell if a person had walked in my room because the inprint on my smooth-combed carpet or WORSE.. the smell of their cologne! YUK!
People breaking into the schedule were ALWAYS resented. even the most favorite like my beloved grandmother or favorite cousin. I could enjoy short visits to their homes but hated them intruding in my structured world.
And you say you 'TAKE HIS SISTER AWAY" without him to give her quiet time. All he may see is you STEALING her and breaking the everyday routine. Which will make you a DIRECT target of any ire.
Behavior modification is the only way. Discipline has no value without communication and a clear understanding. You punish a lion that ate the dog because you fed it a vegan diet. It will fear you. But it will still eat the next dog as long as it's true ned for taurine is not met since cats are obligate carnivores.

I am frustrated beyond measure since the evil pixies replaced my baby at age 26 months. I cannot teach my child to at least merely point at objects he once asked for. Yet I trained a cat to drink from a straw, eat with utensils and chopsticks, and scribble on paper.
And the first 6 months of his not even having connection or separation anxiety with me hurt.
Hurt people take things more angrily and personally.
But as I tell myself every day. I cannot judge his actions on what a less sensory and verbal child would think or do.
Often in retrospect the reason for his meltdown finally comes to light. So simple as the shades were drawn at adifferent time of day or the phone rang more than usual.
Acceptance and affection are all you can offer until some way tro draw them back to society's rules of functioning. But even if he is ever so luckily to stabilize, never assume he is "cured". He has simply adjusted himself to comform. Like dyslexics.
You might try bringing something like bubbles, a balloon, or something that spins like a pinwheel. If he is calm enough, maybe a kaledeoscope. To distract him from th emoment and focus him elsewhere. He own't think of them as "gifts" but your reward is to start the visit drama free.
Good Luck
My user name is NotAllThat Yet

Sometimes when my son has acted this way, it has been because he's trying to avoid doing an unpreferred activity. Basically, he is trying to distract. What the teachers/therapists have decided to do in this situation is to not acknowledge the behavior. That means, no eye contact, no responding with anger, no saying "no hitting!" Do not reward him with a reaction of anything other than calm. What you would want to do right away is redirect his hands. If you can, direct him to a preferred activity. If I were you, I would show up with something highly rewarding and motiviating for him--I don't know what that might be, but it could be anything from potato chips to bubbles to balloons. Maybe it could be something he only gets when he sees Grandma.

Another thing you might consider is giving him (with mom's permission) some melatonin in his juice if it is close to his bed time. This is a natural over-the-counter sleep aid. 5HTP is another mellowing supplement, but it won't have quite as stunning an effect as the melatonin.

He may just be upset because your presence is a transition in his routine. Autistic kids often have trouble with transitions. He may be mad that Mom or Dad have left the house. Mom and Dad can help prepare him for these transitions. One way is with a visual schedule. They can let him know in advance with a picture sequence what will happen: Dinner, Bath, Grandma, pajamas, juice, brush teeth, bubbles, videos, hugs and a song, bedtime. If your visit is just one thing in a calming, happy routine then eventually he should be more manageable. One thing that I do for transitions is that I sing a counting song. Numbers are predictable, they can pass time during difficult waiting times (like for the microwave to cook something) and my son loves them.

I don't know if Oliver is extremely difficult and heading toward medications and an eventual placement outside the home or if he would flourish with just a little bit of concentrated behavioral therapy. Whatever the family decides to do, it probably won't be an easy thing. But I'm sure I'm not telling you anything you don't know.

Your granddaughter might enjoy going to a gathering of siblings of autistic kids. It has to be tough on her, yes, having an ASD brother, but it's all she's ever known, at least since she was 3. This experience is going to shape her, one way or another, and I don't believe it will all be bad. It's wonderful that you are there for her, though. It's a wonderful gift of yourself, time just for her, that you are able to give her.

Please understand that I am not a professional therapist - just a mom with a 20-year-old autistic son who has come a long, long ways over the years.  I have learned a lot from the various therapists who have worked with my son, but this is strictly amateur advice...

From my experience, facial and emotional expressions in people with ASDs frequently do not coincide with what we would normally expect.  This means that Oliver's laughter probably is not an expression of sadistic pleasure but may even be an expression of intense frustration and pain.  (My dad used to have an adage:  "Sometimes it hurts so bad you just gotta laugh"; and sometimes, I found this good to remember whenever my son seemed to laugh out of any logical context.)

My son did strike out in such unexpected ways, but only on rare occasions.  He once whacked me across the face with a toy moments after running up to me and giving me a big hug.  Because these behaviors can seem to come "out of the blue" tracking the "cause-effect" relationship can be a challenging process.  However, the behaviour probably is an "effect" and probably does have a logical "cause."  That cause, though, could be almost anything.  It might be a sensory issue (such as the smell of your perfume, the texture of your skin, etc.).  Or, he may be "trying on" an observed behavior out of context (this is similar to echolalia or the repeating of words without associating their meaning).  Or he may be expressing a more deeply rooted frustration, not even necessary connected with you at all... or he may even have other psychological issues (this is obviously not a conclusion that should be arrived at lightly or without the input of a professional).  

The behavior really needs to be analyzed in depth (usually the realm of his ABA therapist, OT, psychologist, etc.)  In the interim, though, you can try doing an amateur analysis yourself by breaking down the sequence of events and listing the possible antecedents (what happens before things start to go wrong) and try to brainstorm different ways of intervening (breaking the sequence of events) before the point where the hitting starts.  There are several books about autism that describe various methods for doing this.

Depending on "why" he is hitting you, your reaction may also appear to be totally illogical to him.  Therefore, you may also what to change your response; that is, try other things rather than taking him to his room (By your description of events, this tactic seems to only escalate into worse and worse behavior).  Sometimes, I found this the most difficult thing for me to do - to abandon my apparently logical and justified reaction (from my point of view anyway) simply because it was not a reaction that was working with my son.  You could also try "anticipating" the second hand coming and grab it first - maybe redirecting it to a quick "tickle" instead.

I don't think though that his behavior should go on unchecked and should be brought to a professional's attention.  Autism is a challenge, not an excuse to let things go.  Almost certainly, Oliver can learn to express his feelings in appropriate ways - to laugh when happy, to cry when sad, to relax when stressed out, and to respect personal boundaries.  The teaching method though will have to be made to fit his particular learning style (and that is a very individual thing).

Hope this helps.

That does not sound like typical ASD behavior. But of course I am not an expert, I am just stating that from my interpretation of what I have read while starting my journey with my toddler who was recently diagnosed.

Has his mother considered behavior therapy? Is he verbal? Maybe if someone sat down with him and asks why he acts out there could be a solution to this. Some children act out because they are not getting the attention they need, or they feel inferior to another sibling. Things get swapped around in young minds and even negative attention is attention.

Sometimes extremes need to be taken, has it been considered to take him out of the enviroment to a home where he can get one-on-one attention and help until he can manage his emotions a little better. This can be with another relative or foster home.