Expect a 5" fish one day. My watchman goby won't stray more than 5 inches away from his hole - where he vigilantly watches out for his pistol shrimp buddy.
Think about a pistol shrimp for him. If you do, however, make sure your rocks are firmly planted on the bottom and won't tumble when your shrimp burrows under them. The shrimp will give your goby another reason for existence and lmight make him more happy. You'll enjoy the relationship too.
At night, gobies normally hide under a rock and don't feed I believe. Make sure the food you're using sinks to the bottom. Of course my pellets do, and the frozen stuff gets down there also by way of my return flow across the back of the tank to the front of the bottom.