Alright well what kind of water do you use?
What kind of conditioner do you use?
Do you have a heater for the tank?
Have you ever tested the water you use to see what the levels are?
What size is the tank?
Are there any live plants in the tank?
Have you ever used aquarium salt?
You can try feeding him live brine shrimp as they are high in fiber after fasting him for a couple of days. Maybe go to the store and buy some epsom salt try treating him with that for a few days. Other than that maybe try switching up his diet with some bloodworms a few times a week. Good luck!