im 13 and can understand your sons situation. im a 13 year old girl and have lots of problems with bullies but i just ignore them, for the most time, other times sarcastic, witty and smart comments work well,. fighting does nothing nor does swearing, it just makes you as bad as them, reacting as if your better, know more, have more things or are in some way more intelligent is really just stupid and a bit vain. Never hold grudges, if they do apologise accept and move on, they usually need to be coerced to apologise though...
I live in the U.K, and im not sure where you live but these rules are basically universal
Oh, and if violence/death threats/damage to property become's involved, tell a teacher but make sure you have names, after the 2nd time of complaining suspension will usually happen, i wasn't so eager to tell but a nice girl in my year decided to tell a teacher (i didn't know who had told at first) and that really helped to have a witness, for a suspension immediately you usually need, 1 or more witnesses to bullying, names of bullies, person being bullied to tell about what happened, and that's it really 