That is a strange occurance and would initially lead you to think he had s fungal infection causing for the discoloration. Although, if your dont see any white fungus, like shards of cotton or a growth of some kind then the problem may be parasitic or bacterial.
First, if you are feeding live foods I would give that up. Attain some of New Life Spectrum's Thera A that is suited for your fish size, they have large and small fish size formulas - this food is non medicated but can help if there is an internal parasite problem.
Like Brain NY said lots of water changes, every other day if you can/ and add Seachem Stability to the water to cope with the bacterial drops. Id try Mela Fix or Pima Fix if things do not improve, and you could soak the spectrum in Garlic Guard. Sounds like a feeding and enviromental issue to me, so that is where I would start.