Hey Tyler, here is a way that "MIGHT" work, put Rex in a different container for a couple hours with nothing in it. He may pee and go poo in that container. if he does clean the container out and put him back in it then feed him. let him sit after that for a little longer. during this time clean his whole cage. i usually replace the water with water that is 80 degrees and rinse all the bedding with luk-warm water. the cage will be very humid. put him back in his cage. that night he may start croaking. like i said in my last reply to you, it is a weird sound that has a sharp medium tone to it.
this is what i did yesterday and last night he went crazy. It may not work either but it may be worth a try.