This is a tough one, because bullying can be a serious problem with detrimental consequences. These days, the simple schoolyard scuffle of yesteryear is not what's happening these days when fights break out. Often there is nothing friendly about it. Peer pressure and pecking order starts as young as 6, and depending on the value-system these "bullies" are being raised in, the stakes could be high.
Find out what exactly these kids are doing that has your son so upset. I'd be less worried about him growing up being a sissy, and more interested in finding out what's going on when he's away from home. He needs support and reassurance that he can get help if he needs it, moreso than being taught how to survive a street fight with the kids down the street. Maybe it's pushing and kicking today, and a knife or gun tommorrow. No matter what, if it's more than one kid ganging up on him, he should be rightfully concerned.
How well do you know these kids' families? Consider inviting the trouble-makers to your house so that everyone is on your son's turf. Meanwhile, figure out what's really going on (the dynamics between the kids). One of two things will happen. Either the bullies will become "Eddie Haskells" (dating myself again) and will ease up on your son as you become more present in their activities (a way of being a silent ally for your son while getting valuable information). Or, you'll find out who is rotten and can either contact the kids' parents if things warrant it, or make contact with these kids totally off limits because you yourself will know whether your son is being weak or rightfully concerned.
While there is value in helping your son cope with bullies, sometimes its not worth it. If these kids are truly bad seeds, their behavior will only get worse with time. Your son may eventually become rotten like them to cope with the peer pressure, or be victimized by them. Sometimes, giving him the green light to avoid them is appropriate and okay to do. This doesn't teach weakness. This teaches him how to assess peoples values and whether or not he wants to be subject to those values. The bottom line is, carefully evaluate the situation. If it is a minor problem, it will resolve itself. But if he's truly troubled, you should at least give him the benefit of the doubt and emotional support and pehaps takes steps to help him. Afterall, he doesn't have to hang out with these kids only. He can find friends anywhere. In adult life, we select our friends and avoid those we don't like. Why can't he do the same?
Afterall if these kids come from homes where violence is how they get what they want, they're not going to change. If you don't support your son, and come up with a good plan together, he'll be the one left with no allies, and will be the one suffering the consequences.