What are you water parameters and what other fish are in the tank?
This may be a mycobacterium infection, or an intestinal parasite infection. The fact that he isn't eating makes me think it's more likely bacterial than parasitic (though often with mycobacterium fish still have a good appetite). There hasn't been any established effective treatment for mycobacterium, but kanamycin has been used with some success. I would start by treating with metronidazole and praziquantel in case this is a parasitic infection, and then try kanamycin bath treatments.