Have you though about videotaping his meltdowns so his doctors and/or counselors can see what really goes on. It is hard to explain something without having a visual to go along with it. My grandson who will turn 14 next month has autism and has his own meltdowns; however, he has never gotten physical with anyone in our household or at school. It is getting harder for me to get him to do things so I know how frustrating it is. I hope you have a good support system to help you get through this and hope someone gets to the bottom of your child's problems.
I would speak with the consultant that diagnosed him, or the clinical/educational psychologist who has the most experience of him. Learning about emotional control and responses to emotion falls under the Ed Pscychologist. It is hard sometimes to tell others exactly what the situation is like at home, especially if he isn't showing those same behaviours in school, but you aren't doing your son any benefit by keeping quiet. It maybe that his anger and frustration is being kept under wraps whilst at school, and then it all comes out at home because he feels safer there.
First of all speak to them on the phone. Then back that up with a letter telling them that he is physically hurting you when he gets upset. They need to do something about it. It might involve behavioural therapy by a professional that works with those on the spectrum. Or it might involve a change in school. But you need someone to work with your son to improve his responses and not just to move him to a school where all the children have behavioural difficulties.
If you don't get anything positive from doing the above, I would advise speaking with a National Autistic Society in your country, or a solicitor who is specialised in dealing with special needs educational law. They will tell you what you and your son's rights are and that will help you decide what action to take.
What type of school does he go to?