Yellow Tang looking bad?

[Gizmo]

Hey all, I noticed that my yellow tang seems to be thinning out around the middrift, despite watching him attempt to eat lots. Today I fed mysis and he was trying to eat it and the food just sort of fell out of his mouth and he would then proceed to another morsal of food. I'm not sure what to do. I have a cleaner in the tank that I've observed cleaning him at least once a day, so I'm sure there are no parasites.