I was I the same situation as you. I think mine refused to eat anything for almost two weeks. Bought him looking plump, and as days went by he looked skinnier. Well what I did for him to start eating was to place mysis shrimp into the sand with a turkey baster, and watch him. After day three of doing this I finally saw him take in some of the mysis. But I didn't wanna do everyday cuts of water parameters going bad. So I took a trip to the beach and got mussels from the beach where my puffer prefers. Well I took small mussels and chucked them, and placed it where the goby was. I was surprised that he mouthed it. You know they work food around in their mouths. Well I heard they only go through sand for their food. Well my bullet goby is a pig now and eats mussels, snails, conch, limpets, mole crabs. So try getting mussels. I thought mine was gonner too when I saw him get really thin. I hope this helps.