I think the best thing you can do is to talk with your son and let him know people will be rough in life and he is only at the beginning. The best ammunition in dealing with bullying is understanding why it's happening and knowing you (the one being bullied) is not the one with the problems. Any time my kids have been picked on in school or have come home and told me about another being picked on, we talk more about the bully than the one being picked on. Generally a bully will push their weight around because of insecurities. I always ask my kids if they know anything about the bully and their homelife and 9 times out of 10, things come to the surface such as, a mean father, or no father, parents divorced, or mean older sibblings. in the few times my children have been mistreated by another and I have taken that route with them, they have learned that not only are they not the one to feel bad in the whole deal but they have learned to have some empathy toward the bully.....understand ing that the reason he or she is so mean is because he or she is unhappy. Some kids you can reach out to and make friends with and some are just trouble, but either way...if I were you, I would keep an open dialogue with my son, letting him talk to me whenever about anything that bothers him, but I would also encourage him to understand he is not the one with the problem and not allow him to feel "mistreated" to the point of felling sorry for himself, rather, have him take a look at the bully and understand why the bully is the way he is.
I have also pointed out to my children that it takes a fool to do wrong. To do evil is like sport to a fool, but a man of understanding has wisdom.
Hope he understands how great he is and doesn't allow the unhappiness or foolishness of others drag him down!