Put it this way, you are responsible for your child's safety and it's your responsibility as a parent to ensure his care and safety. That includes, when placing him in the care of others, ensuring they are fit people to care for him.
Therefore, write to the chair of governors and copy the letter to the head teacher. Say that unless they can deal with the bullies and ensure your son's safety, you are not prepared to leave him in their care, as to do so is to compromise your integrity as a parent.
If this gets you nowhere, phone the education authority's HQ and ask to speak to someone higher. Either that or write straight to the director of education. You could also notify your local councillor and/or MP. If you get no joy there, your local BBC can work wonders.
You might also want to think about reporting these incidents of neglect to the police. Your son is being assaulted and, as I've said, the school are failing to protect him as is their legal duty.
Basically, make big waves and keep making them until someone comes to your aid.