Melafix would be an excellent choice....use as directed on the bottle. Also you might want to add 2 TBS of salt to his hospital tank.
It looks to me a classic case of rot, which probably happened because of him lying on the floor too long and losing his protective slim coat which allows bacteria to enter.
Is he eating at all? If so I would also feed him a medicated fish food.
Good Luck! Poor fella, he definately need some tlc!