As someone noted, get everything documented. Do not rely on chats in the hallway or even phone calls. Do things by e-mail and keep everything you send and everything you ever get. Send any letters by registered mail so the school, principal, superintendent and school board (yes, write to the school board too) know that YOU know they received the letters. When something happens to your chiild, write down a diary of what happened, the date and time, location, etc.Make sure everyone involved knows you are keeping notes, doing everything in writing as well as in person, and then use that magic word: Police. If you do not get immediate resolution, be sure to say that you are now going to the police. In some cases, as someone posted below, the police WILL get involved so don't hesitate!
Meanwhile -- it is nearly the end of the school year -- is that correct for you? Be absolutely sure you write the school and say you do not want your child in any classes or activities next year with the children who are bullying him now. NAME THEM. "I am formally requesting that my son not be placed in any classes with X, Y and Z in the next school year, due to this year's issues between them. If my child is placed in any classes with these specific children in the fall, I will ask the school board to move my child immediately and will request a report on why this letter was not honored."
Also -- dont' forget in your focus on the school that your son is hurting inside. Please consider whether he might need some counseling over the summer from a counselor experienced in dealing with victims of bullies. This sticks with kids a long time and has even caused some quite young kids to commit suicide. I'm not saying your son would do this, but be aware that kids do not always tell parents the entire story of how bad this can get. Please seek out help for him to cope with this. If you can't afford a counselor, see your county or city mental health department, tell them what you need, and ask about sliding scale fee payments.