man, i'm only 26 and NEVER had these problems! i was friends with everyone and didnt really even hear of any bullying going on at all! WTF is with kids these days??
i'd be VERY careful about your kid kicking the other ones ass. #1, make sure that the bully hits him first, and that there are friends of your son there to witness it. #2, make each hit count, but don't take it so far that the kid is hurt for good. you want the bully to know he messed with the wrong kid, that's it!
if it were me, i'd call the boys parents first. if they're #$%*s about it, then you have to be careful if your son steps up and wins, because then the bullies parents will be pissed and want revenge themselves!!
these days everything with violence is a slippery slope! is there any taunting or is the bully just picking on him for NO reason? usually a bully will stop with just a confrontation. have your son ask him what his problem is in front of teachers and friends, that way he'll get embarrassed and maybe stop! if he steps up, let him push your kid, then have him kick him in the nuts as hard as he can in front of everyone. just have to make sure the little #$%* doesn't try to jump him outside of school or something, especially if they both walk home!
just my $.02