He most likely has internal bacteria infection. Discontinue carbon filtration and treat your tank with Kenaplex (might be Kanapex) according to label ditections. Keep temperature stable around 82 so as not to stress him at this time. Feed well but sparingly, ensuring all food is eaten, and no excess laying about. Be sure to do reccomended water change at end of treatment and resume carbon filtration. If that doesn't work, then we'll consider parasites...
You ever going to post a picture of him/her?