If he has bright green coloring, he is most likely a Green Spotted Puffer, and since you posted this in the freshwater section I am assuming that's the kind of water you have him in.
GSPs are actually high level brackish fish: they need a specific gravity of about 1.015-1017, in my experience. Your fish will not do well or live very long in freshwater.
Also, what size tank is he in, and does he have tankmates?
These guys need a minimum of 30 gallons to themselves, as they are very active and also can be very aggressive and kill other fish in the tank.
(mine, who is only a couple of inches long, killed and ate an algae eater which a petshop owner put in there while he was petsitting him for me)
Owner of one happy little Green Spotted Puffer. :)