Was it at night or after lights out? Puffers are not generally energetic. They will find a corner, ph
, or rock and lay down and just sit there. Some of them and other fish as well, will lose their color after lights out. They become pale and whitish looking(even yellow tangs) until after the lights come back up. They do look dead when they are sleeping. If he is eating, swimming occasionally, not breathing overly hard, and not covered with any lesions or spots, I would not worry about it.