Pictures would be really helpful, and a bit more info if you can.
Do you have a test kit, and if so, could you tell us your current water parameters? How long have you had your tank and is it cycled? What do you feed your fish, how much and how often? Are there any marks on your fish or are there any scales sticking out?
Sorry to bombard you with question marks, but the more info the better the help you get
I'm sure that one of the more experienced members will be able to give you some good answers and advice with a bit more info
Knowing your water parameters ammonia/nitrite/nitrate, etc will help us identify if there are any issues with the water but in the mean time, I'd suggest not feeding him for a day or two to see how he does.