Is he hiding under the paper? How many layers are there for him to hide under?

If the paper isn't thick enough to make it dark underneath that would make sense.

My guy has probably 3 or 4 layers he gets under. And he's doing great.

That or get a good size hide in there so he can sprawl out a little. They're lung is HUGE. And the ability to sprawl out helps combat respiratory infections.

Hope that helped