Oh no! I agree with everyone else. That isn't a pea puffer but a brackish type and I'm afraid a 5 gallon tank is not big enough for it at all... Hopefully you are able to do a lot of research on the type you have and get it the right conditions.
I am no expert on puffers but compared to the pictures I've seen he looks more bloated than most (thicker at the back of the abdomen right before the tail fin). I just seriously love to look at the little guys so I click on every puffer thread.
Overfeeding and bloat often cause "resting" and decrease in activity levels.
This is just general fish observation. Can someone with more puffer experience confirm?