Sorry to hear and I have no personal experience with them but since his teeth are in good shape I can assume that they don't need to be trimmed or filed down?
Have you tried soaking the food in garlic juice? That seems to be a natural appetite stimulant. Crush a clove of garlic in a small amount of water and let the food soak in it for a bit before feeding.
Hopefully someone with puffers will be along to help. Good luck.
Animal testing is a terrible idea; they get all nervous and give the wrong answers.