:( I'm so sorry that your son is going through this. I agree with many of the posts on here, to talk to the teacher. If my son was the aggressor, I would want to know as a parent so we could talk and practice appropriate behavior with others. We have stressed with our son that if he sees this type of behavior to be the one who steps in and stops it - we've practiced phrases to say to the aggressor to make them stop what they are doing to the other child. Since my son is big into superheros, we talk about how a superhero would handle this situation if someone needs help. Perhaps this could be something the teacher highlights in class as well - not singling out your child or the bullies, but as a class practicing how to stick up for someone or intervene in a bully situation, or what to do if you are being picked on?
Keep us posted on what happens - best of luck to you and your son and I hope this stops soon.