the poor thing.
they have very few heat sensors along the bottom half of their body. it would have been well over 130 F (55C) and he would have been sitting there cooking for well over a minuet to burn even at those temperatures. who knows, it could have been much hotter.
they are designed to stay around 15-20C above room temp. (hence giving you a hot spot) BUT, if the hot spot is already 40C its gonna heat up and over 60C.
i surely hope this has snapped you into a mature mode, where we stop selfishly supplying 'easier' or 'cheaper' options for our pets.
use a human burn cream. antiseptic ointments.
keep all of his substrate out, use lightly misted paper towel. and keep the wound clean. once it begins to heal up ( a few days it will dry) then dry paper towel... its just dry paper towel will get stuck to an open wound.
ideally visit a vet. i would have already.
he well have troubles shedding that area as with any other wound.