My heart goes out to you and your son. As a parent, there are few things harder to witness than the torture (because that's what it is) of one's child at the hands of his/her peers.
Why this "Lord of the Flies" atmosphere is allowed to persist in schools is a mystery to me. Where else could some maniac strangle someone else with impunity?
I find it beyond outrageous that the schools, at least in our area, place the responsibility for changing the situation on the victim. The bullied child is sent off to learn how to be more assertive. OMG, no matter what, there is never any justification for physical or verbal violence. It's the bully who requires the behaviour modification.