In the short term:
1. Be sure you put him in his own tank, as he almost certainly will kill anything else in the same tank with him. Absolutely, positively do not put him in the tank that holds the fish listed in your signature, you will likely wake up the next morning to a genocide.
2. If he is underfed, your first priority will be giving him food that he will eat and will help him get his strength back. If he is small, that probably means live snails (pond or ramshorn, ideally...he might or might not eat small MTS as well) and also some live blackworms, if you have access to them. Puffers will not even think about eating flake food and likely won't go for frozen either (unless maybe you try some frozen glassworms or bloodworms and jiggle them around in the water with your fingers to make it look like they are alive).
3. Long term, as others have pointed out, you need a pretty large tank (20 gallons at least very soon, and 30+ gallons down the road) that is slowly brought up to medium brackish conditions and then later full marine conditions using authentic marine salt--not the cheap "aquarium salt" most commonly seen in chain pet stores. Because they need to grind down their teeth, then snails and other hard foods are also absolutely required for long-term health.
If you can't provide all of the things you need to for this puffer, then as much as your heart is breaking seeing him at Wal-Mart, it might be best that you leave him there and hope someone who is able to care for him comes along and buys him.