The biggest thing you can do is go and be with him. He's 12 and needs his father during a time like this.
I wouldn't bother with a guidance counselor. I would call the principal of the school (no reason going up the food chain, start at the top) to let him/her know what is going on, schools take this stuff very seriously now. I would tell the principal that it must stop now before someone gets hurt (not necessarily your boy either). And while you're at it I'd let them know you'll be there in a week or so and if it continues you'll be in his or hers office demanding to sit with the other child's parents to finish this problem. Let them also know you'll be keeping them posted about the progress and tell the principal that they should be present during this time because obviously there's a lack of supervision in the halls.
If he does stand up for himself, tell him to do it when a teacher is nearby and can see what's going on just incase he gets in over his head, after all there is more than one person giving him problems.
If he did go on the offensive, most people will back down including a bully. They're only a bully until something happens to them. Another alternative is to go on the offensive when only one of the bullies are present, they'll probably never stand their ground alone. Pick them off one at a time.
this^