Talk with the school counselor. He may be able to give your son some individual time or include him in a small group on assertiveness training. As you know, there's a big difference between standing up for yourself and being mean to others, and your son needs to know that he can be assertive and still be friends with the other children.
Try to find opportunities for your son to try new things at which he may not be perfect. Joining a soccer team (or some similar activity) will give him a chance to have fun while learning new skills. He will learn that he doesn't have to be perfect at something to have a good time.
If your son's name comes up on the waiting list for the advanced class, think carefully before you agree to place him there. If he is having these problems with stress now, they may worsen if he is put into a more challenging situation.