Ouch!

That is a burn sweetie, and a nasty one. My guess is his heat pad burned him. He needs to see a vet right away so they can start treatment. You will need to keep him on paper towels and keep his tub super clean while that heals up. If you keep it clean and treated, you won't hardly know he did it. But if you don't see a vet for proper treatment, correct the problem with his heat and leave it get dirty, it will turn into a nightmare of a mess.

Best wishes to you both for a speedy recovery!

Gale