Are you sure of the humidity? I have only had tree frogs burrow once and it was because he was dying. I have heard of them doing it when it is too dry. You could remove the substrate and set up a quarantine environment instead. Just use unprinted paper towel on the bottom of the tank that are changed every 1-2 days. That way you can see if he is pooping and if it is normal. Make sure the water is changed daily. Please read through the grey tree frog care sheet - Frog Forum - Gray Tree Frog Care and Breeding