They are one of the more social puffers. I have heard many many many people who kept them say that there teeth grow faster than most puffers and may require manual teeth trimming. I've heard others say if you just keep them fed a regular diet of snails there teeth will be fine. Puffers prefer live food but will accept frozen foods. I would suggest Blood worms either frozen or live and try to get a small tank started for a snail tank. Id feed him baby Golden Mystery snail baby's, Baby apple snails and other things like that. Until you can get those, you can feed him Malaysian trumpet Snails. Your LFS will have them EVERYWHERE in there tanks, or at least mine does, just ask if you can get a few out. Id feed him at least 2 snails a day and a small pinch of blood worms every other day.
Now all that being said, i have heard some people say these puffers like being in groups of 5 or 6 individuals however your 15 has no where near enough room for 5 or 6 puffers. I don't think your puffer will mind being alone. Decorate the tank very well, puffers are super curious and if he gets bored he will begin roaming the glass, going up and down and up and down all the time. Make lots of little spaces for him to roam.
