Someone made a post similar to this awhile back, and what your describing sounds like what they were. People felt that it could be Blister Disease. Here's a definition from Peter Kahl's website:

"Ball pythons are native to very warm, but not hot, dry areas in Africa. Many people make the mistake of trying to keep them in a too humid overall environment, using damp sphagnum moss or misting them frequently throughout the day. The problem with this is that keeping the overall environment damp leads to conditions such as blister disease where in the skin, usually of the belly, becomes covered with blisters, leading to bacterial infections of the skin, which in turn leads to overall health problems." ( http://www.pkreptiles.com/informatio...allpythons.asp under heading "Humidity and Ball Pythons")

I'm not sure if that describes accurately what your snake's showing--but I'd definitely get him to the vet.