Scale Rot?!!?

Definitely consult a vet, ASAP. That looks like an extensive burn, and it might be necessary to put him on antibiotics to prevent a serious infection.
And get a thermostat for the heat pad, so that can never happen again.

Caring for a ball python takes $$

Thermometers? why weren't they in use?

Thermostat, why no thermostat?

Sad, I feel for the snake.

Poor snake. Looks like a burn to me, but I have never seen anything like it so take my comment with a grain of salt.