Pufferfish are hard to care for. They're a brackish water, scaleless fish with a low tolerance for stress and disease. Making matters worse is the fact that big retailers stock puffers in freshwater tanks and don't tell you that they'll need to be moved into a higher salinity tank once the fish reach a certain size. The result is that they're often stressed, ill, or both by the time you get them home. Being a scaleless fish, they're very sensitive to temperature fluctuations, contaminants, and ich treatments. Too many times have I seen a spotted puffer die because of the poisonous effects of products like QuICK Cure on their fragile biological systems.
Make sure your heater is working properly and check your salt level. It probably would be a good idea to move the sick one to a quarantine tank in case he has a communicable disease or parasite. Ich, for instance, works fast; in as few as three days, your fish could be host to many parasites that are busy making more parasites to infect your other fish.
I recently bought six spotted puffers and had to watch all six of them die slowly of ich because the treatment is often worse than the cure. Additives like StressZyme are okay for your puffers and may help the sick one recover. Keep your fingers crossed and do what you can to keep their stress levels low (avoid tapping on the glass, disturbing the landscape, etc.) I hope he recovers and good luck.
