My middle-schooler is the victim of some pretty vicious gossip. How can I help him?
Sticks and stones can break your bones, but words can tear right through you. First let him know that you take this seriously; that it's not simply, "Oh, never mind them, they're just jealous," or "Don't let it bother you." That's not nearly enough.
Second, find out as much about the specific situation as you can, and then decide if it's best to confront it. Sometimes it really is best to go straight to the source and say something like, "I've heard that you've been talking about me behind my back. I'd really appreciate it if you'd stop doing it. I'm really not that bad a person when you get to know me." It takes guts, but it will often shut them up. (Sometimes people are so mean that confronting them might actually make the problem worse, so be careful with this advice.)
Third: As difficult as it is, help your son live above it. Remind him that as painful as it might be, this is a temporary situation, and like a wildfire it fades fast if it's not fed. Also remind him that what goes around comes around: if you give venom, sooner or later it will come back to poison you. That may help him resist the temptation to retaliate and perpetuate the situation, and it may bring him some comfort knowing that his tormentor will (eventually) get his comeuppance.