How much and how many times you feed him a day? You may want to feed him some pellets which have been softened with water, and feed him twice or thrice a day, each time in small quantity to ease his digestion.
Have you checked his body complexion? Is the body color consistent or are there any irregularities such as patches of distinct color from the rest? You may want to check for any parasite or bacterial symptoms.
Where does he spend at most of the time? How does he swim, smooth or rather awkward?
You may check on his gill flaps activity, are the flaps stuck open most of times?
I suggest you perform daily water change at 90-100% and apply some stress coat on him. Don't forget to test the water chemistry and apply conditioner.
Btw how is he now?