I encourage you to make an appointment with the school counselor and bring this issue to her attention. Find out from the counselor if the school has a buddy program, a friends group, a peer-mediation program -- some way to help him fit in at the school. Keep in contact with the counselor and evaluate the progress of the plan for helping your son. Be firm. Don't tolerate continued harassment. If it continues, seek out the person in charge of discipline. Again, calmly but firmly insist that the school take action. A final step, which can be hard to take, but is necessary, is to file on the students with the Student Resource Officer or with the local police.
Encourage your son to think about some child he knows who does not pick on him, or another child who is also not accepted. If possible, invite that child to play with your son at your home. Above all continue to show your son love and encourage him to ignore and not believe the put-downs and insults.