I don't know if you already have a solution in mind, but here are what some other people are saying and or doing:
http://www.bridges4kids.org/SC.html
Personally, I think you should let your son figure this one out on his own. I know it probably will be an unpopular opinion, but it's a lesson that has to be learned (how to deal with bullies) and it's best to learn it now when the stakes are much smaller. Things get much more complicated in adolescence, or even college, and the stakes are much higher.