I would keep your fish separated as I suggested. Up the tank temperature to around 82and add melafix. Keep the tank free from stress, i.e. don't turn the light on and off several times per day. Even though the damage hasn't hit any vital organs, it is still extensive and prognosis doesn't look good. It's a good sign that he still has most of his fins, so that could help. Keep us updated please on what happens with him.
As for the rest of your piranhas, I would suggest getting out of the piranha business. Personally I don't think they are meant to be kept as pets and they can be downright dangerous to those that aren't qualified to keep them. Since an accident like this happened to yours, I think it's time to turn them in to the local fish store or aquarium.