Nice. That loss of appetite is also a symptom when it comes to internal parasites. Obviously i can't be sure, and i don't mean to stick my nose in your business. It's just my 2 cents obviously, but having seen infected fish for years while working in a fish store, and privately, i am pretty confident that this fish infected.
Something that is very easy to use and works every time are Sera bakto tabs. They are cheap aswell.
Good luck with him, and please keep us updated with this future beast. I'd really love to get my hands on one like yours.