My Son, at about the same age, was bullied at school. I talked to him as an adult, and told him to view these kids with pity, because it is them who is lacking in confidence, trying to boost their own egos by bullying. If you show him that they are in the wrong and that their behaviour is despicable, but don't let the other kids know that it bothers him it may be easier to accept. Don't make the mistake of saying 'fight back' with physical violence, talking is much better. It may be a slower proccess but, for my Son it has been very effective. He is now in the upper 6th form and is very popular. Which proves to me that word power is real power. Kiss X, for your Son, and make sure you tell him how much you love him and that he's not on his own in this; you are right behind him, supporting him. It worked with my Son. All the best Little ducks.
Caz x