What could be wrong with my cat (any vets here to help me out)?

I'm not a vet, just a retired vet nurse, but it sounds like your cat got some sort of injury in the fight and it's probably infected by now or maybe an abcess. The vet will need to have a good look at your cat to find out so it's good you are taking him soon. It might be a good idea to ask about having him neutered too so he doesn't go out fighting.