Children can be so cruel to each other, and although some kids come throught the fire stronger, others can have their spirits crushed. I was one of those kids who always got teased and it had an effect which lasted long after the events faded into history. I think it is very important, more than anything, that a child feel that his parent understands and supports him, even if the parent is not able to control the problem.
I also went through an experience with one of my sons when he was in junior high school. He became the target of bullying from another student, who tormented him at every opportunity, encouraged others to do it too, and even tried to provoke fistfights. This bully it turned out was the son of one of the psychologists at the school. She and I had a conversation after the fighing incident, and we determined to get together with both the boys, and discuss the problem. Although the bully was far from cooperative, it seemed that just some good did come from the talk with all present, and there was a lessening of the trouble.
You might think about getting together with the other kids and parents to talk, that is if the other parents can do that without trying to invalidate your son's pain and downplay the role of their own offspring. Another thing which might help is to arrange situations in which the boys can have the opportunity to become friends outside of the usual circumstances. Perhaps the bully could be able to see your son as a human being rather than a target.
truman