My favorite! I used to take care of pea puffers myself, and I usually crushed snails (Ramshorn) to feed to Puffy. (His name) I fed him from tweezers, and when I crushed the snails, it also reduced the pollution in the tank, the teeth for the puffer was trimmed short, and he got his nutrition. I also fed him fresh bloodworms, (midge fly larvae) as a treat in summer.
Don’t feed dried foods, for me, only pest snails and live food will work. Keep a separate tank for specifically breeding Ramshorn snails, and I think he will be fine. Can you send a picture? I MIGHT (I probably won’t be able to) be able to see what’s wrong. Please tell me updates about your Puffer. 😉