My snake had a burn - he burrowed to the glass one day which he never has done in 2 years ... I had the UTH on the outside of the tank, but it was unregulated (my mistake) - his burn wasn't as deep as yours and he needed injections every 3 days and a cream to restore the skin etc. There's a chance this gets infected on the outside then your looking at spreading to an internal infection. I would get him to the vet asap as you said. He's fully healed and back to normal now, with some scarring in the bad spots.

I wish you luck