You can add more hiding places to make him feel more secure. Are there any other fish in the tank? If you don't have any plants, then I would recommend leaving the light off for him to become more comfortable. You can try offering him cut up pieces of tilapia fillet, that's what got both of mine eating after I got them home. Do you have a pair of feeding tongs? They hunt by smell, so you will have to wave it in the water to get the smell to spread, then hold it still when he comes out looking for it. Don't try to force it in his face, that could spook him, and make him not want to eat. I would try feeding him right after the sun goes down, or 5 minutes after you turn all the lights off in the room, to mimic his natural feeding response.
Get that water tested asap, if there is any ammonia or nitrIte, he could be stressing out and getting sick due to that. If the nitrAtes are very high, then that will also cause him stress/disease/going off feed.