My son Logan

After not knowing what to name my sons issues and behaviors I finally got help through the school and in so doing I got the doctors to evaluate him, they finally told us that he has Atypical Autism. I saw something different about him and so did his teachers but we couldn't put our fingers on it. It is nice to know it wasnt all in my head and now I can do something to help him. He has problems at school. He is a twin, he has a twin sister and they are 4 yrs old. After testing at school they found out developmentaly he is a year behind her and his class and he has a IEP and therapy but I have a very hard time getting him to concentrate and do his homework, If I can it takes a very long time and I also have his sister to help too and between them it takes quite alot of time to get it all done. I wish I knew of things I could do at home to help him catch up the things the school told me to try aren't helping  and they didn't give me a lot of options to begin with. Other troubles at home are that he has a lot of tantrums and I cant really sooth him, he has a hard time understanding me and I have a hard time understanding him. I feel lost alot of the time and after the diagnoses I dont really know what to do from here. He has an IEP plan at school now and all the teachers have been very helpful and kind. I just have this feeling like I am supposed to be doing something else to help him but I have no idea what else to do. After your kid is diagnosed with a spectrum disorder, what do you do afterwards? What needs to be done? What services are you supposed to look for? What help do you need or have to seek? What are some things you can do at home to help them learn and catch up. He does't quite know his number and letters. If anyone can give me some advice it would be extremely appreciated.