Take him to the vet immediately. Perhaps you should take the snake to your home where you can more closely monitor him.

If you cannot afford the vet (I understand Christmas is two days away) replace the substrate with dry substrate immediately. Put a tiny amount of neosporin on the wound and wrap gently with vet wrap. (They might sell reptile safe burn and wound cream near you.) Be sure to very gently wrap the wound with vet wrap, or wet substrate can get in the wound and cause an infection. Please keep us updated.