I would say he is a bit on the lean side, but not too emaciated to lose hope yet. the entice product is good, smells like bananas. i would leave the light off on the tank, an dmake sure that the puffer not only has rock but CAVES puffers need houses and if he does not have what resembles a "dog house" to hang out in he may be a bit stressed.
let the fish store know what is going on so that you may have hope of redeeming some credit if all of your efforts fail. was he eating at the store?