Ask the counselor about the process for having your son tested for the school's program for gifted children. He may qualify for extra help that will challenge him more than the regular class.
Talk again with the principal. Let him know that your son is still being harassed and bullied, and that you expect him to make sure it stops. Write a letter to the principal about the kind of teacher you want your son to have next year, as well as your expectation that he will not experience the same problems with his peers.