I'm sorry to hear your son is having this trouble at school.
I don't know if you've already found these websites, but there are some good online resources with information for parents, students and schools on how to stop bullying.
Bullying no way
Bullying - Everybody's Business by Kids Helpline
Bullying in Schools and what to do about it by Dr Ken Ribgy
Friendly schools and families
National Coalition Against Bullying
State education departments have pretty clear guidelines on what schools must do to stop bullying. If the school is still not helping you adequately you might want to read up on the relevant policy so that you can take it further if needed. I really hope that you, your child and the school can work together to fix the problem, because no child deserves to be miserable at school because of the nasty behaviour of a few.
Good luck with everything.
Annabel.