Kinda looks like the beginnings of scale/belly rot. Has the snake been soaking a lot or is it's substrate wet most of the time?

I'd get it to a vet, first off, to make sure what it is. A short term treatment for scale rot, however, is this: clean the area around and on the affected scales with a cotton swab dipped in 50/50 water/hydrogen-peroxide. When it's thoroughly cleaned, rub on Polysporin (like neosporin... found in the same isle). Make sure you rub it into the scales. You have to do this---clean and polysporin---twice a day.

Again, though, I'd get him to a vet... that's an awfully large affected area.