One of the best foods for that fish, if you can't get their natural diet of live crabs is earthworms. I feed them to all my fish occasionally. If I don't collect them myself I buy them in Walmart for a couple of bucks.
I feed them for their gut bacteria which I feel is the secret to being successful in this hobby.
Porcupine puffers need something with a shell like snails or in time, you will have to trim their teeth.
A big problem with them is that they get as large as a basketball so won't live long in a normal home tank.
I have seen some very large ones in the sea.
I collected this guy in the Atlantic and kept him for a year before giving him to the Coney Island Aquarium as he grew huge.