Have there been any significant weather changes in your area recently? Sometimes weather can effect a frogs mood or activity level.

From what I know it seems lots of people keep more than one dumpy, I'm sure you could introduce a friend and see if that perks him up a little bit. If he's still active and eating and there are no physical signs of distress, your frog is fine. Just monitor his eating habits and activity level closely and see if there are any significant changes.

My White's will go through very active periods and then times when they slow way down and don't move around much for a few nights. I think this is normal.

But Rae is definitely right, make sure your enclosure is big enough (20 gallons for 2 White's being the absolute minimum. It'd be best to go with a 30 gallon or higher since these frogs get so huge in adulthood) and that you do not mix species.

Good luck! I hope your little guy perks up soon!