You need to write to the principal

I don't think this is a small case, I know American schools has "zero tolerance for bully".  You need to write a letter to the principal, but in your letter don't focus on your son, you need to focus on education for bully.  I don't think your son is only one got bullied or that person is the only one likes to bully others.  Another way you can do it is to email to every parents in your son's class.  My son's class has a Room Parent, she is the one usually email us what we need to bring for parties, for the teacher.  So you can email to her or just email to every parent.  You need to bring attention to all parents, and use their power, voice to speak for your son, speak for the bully actions.  Good luck to you!  At the same time you need to teach your son how to stand up for himself.