I see some very small gravel pieces in there....any chance he ate one and is impacted/trying to pass the gravel? Move him into a barebottomed set up and try feeding VERY soft foods such as chopped earthworms to help him pass it.

Any more clear photos from behind or above?

I definitely recommend getting rid of that gravel and doing barebottomed while he is like this...it must be very tender and sore.