He has been burned again. When you take a temperature reading on the paper there's enough air between the sheets to act as an insulator. When your snake lies on the paper that air is squeezed out and your snake gets burned. Why did you not put a thermostat on the heat pad? They only cost $25-50 to regulate one appliance and would have saved you a vet bill.
I'd bet he is biting because he is in pain. Unplug the UTH and keep it unplugged until you get a thermostat for it. This one on Amazon is decent:
http://www.amazon.com/Zilla-Reptile-...sr_1_1?ie=UTF8
In the meantime treat him for the burns as you did before. If you need more silvadene you can buy it online at http://www.mountainside-medical.com/