Looks like what my rtc got before. Mine was in pretty bad shape too, so don't get to nervous, mine came back very quick with the right treatment.
I put him in his own tank to start off. I went to Petsmart got got the Mardel for ammonia burns I think it was, or body fungus, Just open the box, pretty sure the packets were blue that have API and in the middle of it say tetracycline. I did a 40%water change everyday and the water I took out I made sure that much meds was put back in. You do have to half dose it. Don't forget to remove any carbon in the filters because if you don't, it will just take the meds right out of the water. That did the trick with mine, and it looked just like yours does, just not as extreme.
Good luck. I'm sure he will pull through.