Great, sounds like your water quality is great and the tank is cycled and other fish are doing well. I'm not the best at diagnosing and treating, but if I were you I think I'd try to feed him see if he eats and if he doesn't get a lot better or has stopped eating then you'll probably need to medicate. A lot of times when they have swimming issues its because they have injured their swim bladder, but that is more like having troubles staying upright and swimming forward etc. Lotus has linked to a disease database a couple of times that might help you diagnose a problem... I can't seem to find it right now, but if you browse in the disease forum you'll probably run across it?