These guys are notorious for not eating, my opinion it's likely internal parasites. Is he in a qt tank, I would not wait or hesitate to start an internal parasite treatment regime. My method is prazi pro dosed into the tank, this sometimes alleviates the load of parasites enough for the fish to start eating. I usually do 2 rounds of pp. When he starts eating I mix seachem metro/focus into food and feed laced food for 10 days or so. The combination of these 2 drugs address a broad spectrum of parasites and worms. API general cure contains both of these drugs, I don't have direct experience dosing this into the tank for a fish not eating, many use this drug mixed in food.
You really do have to act quickly, whatever you do, once you see the white stringy poo or pinched stomach, it will likely be too late.