...has it got blue dots on it's cheeks...?..If it is then you will definately need to supplement it's feeding...The Sleeper Blue Dot Goby feeds off the bottom as it sifts through the sand eating mouthfuls of substrate and the food that lives within the sand. The diet should include a variety of live and frozen brine shrimp, mysis shrimp, live black worms, and prepared foods for carnivores, and it should be fed frequently.
nutrients in your sand are probably exausted, I would put him in a tank that does not have such a goby. He will sift the sand in that tank and in that way survive, it will also give your tank a chance to build up some nutrients. You should also try to get him on some kind of prepared food but that can be difficult, especially now. You don't have much time left. Get him into another tank ASAP, or get him some other sand.