I would feed him any or all of the following, depending on your comfort level with buying the things. I use the following for my Puffs in the following cycle.
Frozen Krill and Squid (alternated each day- one feeding per day).
Frozen Formula 1 & 2 Cubes cut up just as they start to defrost (alternated with the above).
Frozen Silversides (couple times a week- I cut heads off phew!)
Tetra Min tablets ( 2 or 3 each couple times a week at night as treat and for them to chew a bit for those muscles and teeth).
Mine are too big to care much for worms, brine, mysis shrimp so that goes to their tankmates who enjoy that along with their worms and flakes (Puffs normally do not eat flakes).
If you see it eating anything at this point just give it a fair amount of that till it is back on track then work on hard things to chew. Altho, I do not believe in live food a few smaller snails will do just fine for to chew on or even a small clam with shell (this is what they like to chew on in the wild.
You have my best hopes for your Puffs recovery and good health. Scotty beam me up my job here is done for now lol.