Carlos has given you some great advice.

I think if you can get the temps proper, keep the humidity good, get a smaller water bowl, add a cave or smooth/safe plant to hide under, darker your tank sides to a nice green or brown, keep him in a nice quiet, low traffic area, and let him adjust with less stress, he should start eating. I would even add a bit more substrate so he can burrow up to his eyes. Then he can relax .