What you want to do, is "move the substrate for now" and measure the temp on the Glass/pvc/bin Whichever hes in. Then add about 1/4 to 1/2 of substrate. Reason is, if at the top its 90.... the glass/pvc again will prolly be 95+ lot of heat loss on loose substrate. Even thicker paper i loose about 2-3 degrees.
He will burrow/dig into it, and usually rest on the cage istself to get to the heat.