He probably has parasites. It could be anything from ich, to velvet, to bigger ones you can see with the naked eye.
What are your water parameters? And get a good look at him. Does he have white dots on him, as though he's been salted? Shine a flashlight on him. Does he look as though he's dusted with gold or copper? Do you see any tiny things hanging off him? Anything else unusual about his appearance or behavior?