If his substrate was damp, it needs to be completely removed and his cage scrubbed down and cleaned. Those blisters of his also need to be taken care of. Rub them down with hydrogen-peroxide mixed with water (half and half), them rub the blistered areas with either Neosporin (with no painkiller additive) or Polysporin (polysporin is better for a snake). Do this a couple times a day and he should shed the blisters off.
Information for treatment can be found here:
http://www.redtailboas.com/at_home/at_home.shtml
I know it's a boa site, but it's got lots of information. If his belly gets any worse or it looks like it's spreading, take him to the vet... he may need to get antibiotic shots. Hope this helps!
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1.1.1 Ball pythons (Diablo, Mercedes, and Ciqala)
0.2.0 Cats (Kate and Clarice)
1.0.0 Rabbits (Milosh)