Good for you for calling the schools. Bullying for any reason should not go on or be allowed. And, there are laws that insist that schools won't allow that.
And as Mark said, self esteem issues might be a problem. You might look into buying "Feel confident" or other books listed below it and read it to your son. Check out this link - http://www.amazon.com/Feel-Confident-about-self-esteem-Series/dp/1575424428/ref=pd_sim_b_15?ie=UTF8&refRID=142E95MD25G320Z1P1M2
There is also, "When I feel afraid" http://www.amazon.com/When-Feel-Afraid-Learning-Along/dp/1575421380/ref=pd_sim_b_11?ie=UTF8&refRID=1BJ2NMS0GXQTNKYZDWWK
Or, "Talk and work it Out" - http://www.amazon.com/Talk-Work-Out-Learning-Along%C2%AE/dp/1575421763/ref=pd_sim_b_9?ie=UTF8&refRID=1ND5C2BZN9KHM9RZVPC7
Hi Alexis. This sounds like a self-esteem issue. I think that discussing these situations with the school is the best thing. If there are self esteem issues, you may want your son to be counseled by the school counselor, psychologist, or social worker. Or, perhaps you can seek private counseling independent of the school.