DS, who's nearly 15, had a fight one on one with and old enemy. While the two were on the ground four or five of the other guy's friends went in and kicked DS. There's film of this which another Dad has told DH about. Apparently one boy runs in and kicks DS's head 'like it's a football'.
DS has a swollen face and eye socket. He's been too A&E. No break or fracture on the eye socket but the nose is probably broken. Too swollen to be sure at the moment.
I want to go the police. There is film of the incident. DS is going ABSOLUTELY MENTAL at the prospect of my reporting it saying it would ruin his life and he wants to deal with it in his own way.
It turns out 'his own way' is getting older, harder kids to go and 'have a word', scare the shit out of this kid and probably beat him up. Putting petrol on the flames in other words and using violence on violence on violence on violence on violence on violence. 
Another possibility is that we could show the film to this other kid's school. The kids kicking my son are in school uniform though it didn't happen on school grounds.
I feel completely out of my depth. I have a 9 year old ds who looks up to his 15 year old brother. I don't know about violence. I've never been in a family with a violent person in it before. I'm very upset and confused.