Any suggestions.....

Ok I have a 4 year old son with Asperger's. Normally he is very sweet and loving. But today at school he got in serious trouble for hurting another student. Story behind that is the student has been picking on my son since school started because my son is very small. I have notified the teacher several times about the issue. Well apparently today the kid started messing with him so he took matters into his own hands. This is the second time he has gotten physical with another student. The first time was because he was being picked on by a different student. My question is how do I get him to resolve the issues with the bullies without him jumping to getting physical?? I have been able to handle all the other things that trigger my son. Except this one. I'm at a loss!!