As long as you don't beat him up too much to get him to call the authorities all is fair in there. I'd suggest punches to the stomach because one they don't cause any medical bills unless you break the ribs, which I wouldn't suggest because it could cause internal bleeding if hit hard enough. Try hitting the diaphragm or his lower region and this brawl will probably be over considering that he's a small guy it won't take too long. Also you might consider making him start the fight so that the guy who threw the first punch gets nabbed rather than receiving the blame for something stupid.