What an awful thing to have happened.
I presume your son has a diagnosis of being on the autistic spectrum?
If I were you I would keep my son off school for the time being if that is feasible and if it does not prejudice your case.
I would send a letter into the principal stating exactly what you saw and demand an explanation and an investigation.
I would also find an advocate who specialises in educational law.
I would start by contacting the Autistic Society of America to see if they have information on advocates that specialise in Educational Law. If they don't, I would get in touch with the professional body (something like the Law Council) that will be able to give you that information about its members. I'm sorry I don't have that information as I am in the UK. Maybe you could send a private message to SueNYC as she might be able to tell you where to get this information. SueNYC is usually in the aspergers or autism forums.
Your immediate reaction is to sue, which you may go on to do. But remember that this incident is an indication that the school cannot cope with your son and had resulted to totally inappropriate means of restraining him because they didn't have the staff available with the right training to deal with whatever the situation was. This incident might be the golden ticket to getting him into a private school that has experience and expertise in autism.
Let us know how you get on.
Contact the police and find out what they can do. Sueing the school is not going to accomplish getting rid of the teacher. Well, it might but it would probably take longer... sueing the school an option, but you have to establish stuff was done wrong, which I don't know too much about the law, but to me, that usually would mean contacting the police department and documenting what happened. Another thing to do would be to contact the local newspaper. Have a story done on what happened. What happened was totally unacceptable. If newspaper won't buy the story, contact a radio station. Man, get the word out, embarrass the hell out of the school and get someone to take action. Sometimes TV stations will help you with other things that need to get done. They could suggest legal actions you could take and how to take them. You getting a legal action is also in their interest (it means a juicier story for them).
Have you taken your son out of the school? Are you able to send him to a different school? You should force the public school system to pay for his transportation to another school or get him into a different classroom with a different teacher.
Oh, another idea is to contact your state representative (congressman). They usually help people with how to go through the right steps. You usually can find government phone numbers in the phone book. Basically ask for all the help you can get. Someone is bound to figure out how to help you. Police, state representative, autistic society, newspaper, somebody.