oh a little spotted lol. is he is brackish water? and they don't get massive so snails is about your best option for things with shells still on. shrimp, clam, crayfish etc are good options as well.
Get some cockles or clams from your lfs, then
Partially crush them with a spoon before putting them in the tank. When they are little they can't break into them but you still get the benefit for the crunchy food as they will eat the shell anyway. This worked for my fahaka from about 1 inch and above.
Brine shrimp, bloodworms, tiny ghost shrimp and my personal favorite, snails from my main tanks sump. The hard shells from the snails will help "trim" the puffers beek down.