That sucks. I'm sorry. I guess for girls it's a little bit different. The girls are just as vicious (maybe more so - girls can be REALLY mean to each other), but with girls it's easy... she who wears the most designer clothes wins. When I was a kid, I was from a lower-middle class family, on scholarship at a fancy private school. Of course, my homemade clothes could in no way compare to the beautiful, expensive clothes my friends wore. Yes, they made fun of me. I lived through it, but for a long time believed I was ugly because of the mean things those girls said to me. Anyway, now that my girls are in school, they are always (ALWAYS) dressed in the first stare of fashion. And what do you know? Instant popularity. It's utterly bizarre.
I don't know what the equivalent is for boys? Sports, maybe? Does your son play Pop Warner football, or soccer, or anything? If nothing else, having an outside focus might give him an outlet, and remind him that no matter what those jerks at school are like, he's still got a bunch of friends from football/soccer/whatever. I don't know... but I can sympathize, and hope you find a solution.