Honestly, mine eats anything. He eats pellets, finely chopped table shrimp, nori. Puffers have teeth that are always are growing. Things like shellfish will keep those teeth from overgrowing until they cant eat. Mine wears his down picking around the rocks for leftover pellets though.
I have read clams too but what do I do just get frozen clams from the grocery store and put one in the tank? I'm not sure what to do. The fish was a gift I don't know much about them. Doesn't help I'm new to this
Honestly, I wouldn't worry much about him. They are very low maintenance fish. You can just get the cheap frozen shrimp from the store and chop it up in small chunks that he can manage or mussels and clams in the shell. Mine eats right from my hand. I don't like feeding anything that looks like my expensive display clams. Mine is a model reef dweller and I don't want to peak his interest in them, Lol.