Most of the advice here is pretty salient.
Don't ever, EVER, be the start of a conflict. The very best thing you can do is have multitudes of credible, unbiased witnesses when and if things get dirty. Very clearly, unambiguously be seen as the one who did NOT instigate.
However, I have a very low tolerance for physical violence and I hate with a burning passion the small minded wastes of carbon that bully and torment others in high school. I've been through that, and I won't go into it on a computer forum, but suffice it to say you have to have a stiff upper lip and realize where the high ground is.
In a physical confrontation, what has been stated previously is absolutely true: strike immediately and with full force, without regard to what position it puts you in. Everyone has a Hollywood idea of what fights are like. They seldom last long, and it generally is not a Fight Club exchange.
No one messes with an insane person. So, fight like it. Go insane. No limits, no reserves, nothing held back, nothing that says you are anything less than willing to eat this person's internal organs. Nearly all animals and sentient beings alike realize when they've lost, and then the escape panic hits. Your goal is to put this fear into your opponent, and continue mercilessly beating them until you are tired, or the cops/paramedics show up.
However, reality is a bitch, so here's a few things to consider.
As asinine as this sounds, do not do permanent damage to someone's eyes, as this seems to fare poorly for those who do in court. Do not punch someone purposely in the temples, as this can cause comminuted skull fractures and laceration of the middle meningeal artery, causing almost certain death unless recognized by a physician promptly. A strong hit to the orbit (eye socket) can cause orbital blowouts, many of which can cause permanent visual damage. Hitting the nasal or lacrimal bones and drive bone fragments into the frontal lobe, and striking to the throat and cause collapse of the larynx, leading to asphyxia.
It is, however, very difficult for all but the strongest persons to cause liver perforation, splenic perforation, or GI ruptures with a simple fist beating, although kicking with boots has been shown to.
Just a few thoughts.
Give 'em hell if he asks for it, but go to all lengths, at your wits' ends, to avoid this circumstance.