He looks emaciated, poor guy. Sometimes sleeper gobies starve to death even if they're eating well, possibly due to internal parasites, or it might even be caused by something else -- if the fish are caught with cyanide, for instance, they may waste away over time and there's really nothing that can be done.
I've had sleepers in the past, and always used a long pipette to inject meaty foods (large quantities) under the sand, for the gobies to sift out. The first pair did very well, staying nice and fat. They were killed by an aggressive pair of clowns (by the time I knew what was happening it was too late), so I bought another pair. This time, only one goby survived, despite the same feeding regimen. The remaining fish stayed fat and happy, long after his mate starved to death -- she ate like a pig, but wasted away despite everything I tried. I soaked their food in PraziPro, fed them high-quality foods that included lots of roughage (crustaceans like mysis, cyclopeeze, etc.) and then tried metronidazole (antibiotic/antiparasitic). Nothing worked, poor little thing.
I wish you the best of luck, he's a beautiful fish and I hope you can fatten him up.
