Every sharp nose puffer I have known....and I used to collect a few in the Keys back in the day were pigs. Of fresh caught fish...they were almost always the first to feed. One of the most intelligent btw.
Two thoughts. How old is ur tank? Do you have well develop ed live rock? Small snails? These guys are VERY good at finding juicy morsels in your reef. He may be just very well self fed. You might experiment with finely chopped fresh or frozen shrimp.. clams... or scallops. The are easily hand fed. You can use an appetite stimulate or medcine (see below).
They are fairly easy to catch so, if above, doesn't help, put him in a isolation tank. Dose with prazipro. This very effective on internal parasites. Be sure to follow a the complete course of treatment. You can medicate his food and the water. Be warned prazi can reduce, temporarily, his appetite. So the course of treatment is usually split into at least 2 blocks of 5-7 days.
Good luck the little guy is likely to become one of your favorite pets.