write the principal a letter with the details of your son's bullying. Make sure to send the letter certified, return receipt requested. Obtain a claim form from the school district and file a claim as well. Then, if this happens again, you have given them actual notice.
I would suggest you sit down with school administration now. If your concerns are not resolved to your satisfaction, consult with a school rights lawyer. Good luck.
The diagnosed conditions and medications impair your son's credibility and, thus, he has an extra burden in my opinion to avoid physical altercations. These altercations need to be prevented. Bullying is now in the public eye as a problem; so, you'll probably make headway with the school administration. Review his I.E.P. before meeting with the Principal; maybe you need to meet with someone else/additionally.
Suing the bullies or their parents is a possibility, but it's probably not worth it and will be tough to get a lawyer to help.
It's better to involve an education law attorney who can go to the school and get things sorted out.