Since you've tried all these meds, and nothing has completely knocked whatever this is out - I would now suggest a different approach. Remove all meds from the water, and just wait & see what happens. Maybe it's a negative reaction to the meds that's actually causing all the problems. Or maybe this is something which the fish's natural immune system has to fight off on it's own, or it kills him. Every now and then you'll run into something that's just not treatable.
I would just QT him long-term, feed him plenty and hope for the best. Whatever is wrong will either get better in time or it will most likely kill him. Either way, what do you have to lose at this point?