Brazilian Rainbows not particularly susceptible to dermititis (scale rot), but it starts as discolored ventral scales (pink, to red-brown) originating from the animals vent and spreading to the rest of the ventral area if allowed. I might be able to help more if you could post or email me a color photo of the infected area. Also, detailed accounts of your husbandry practices....heat and humidity to be specific.
It would be a good idea make sure three things happen right now. Seperate him from any other reptiles in your house if you haven't already. His temperature gradient should be from about 74 on the cool end to 86 on the warm end. His humidity should be kept between 70-80%, and he should have a water dish large enough for him to soak his entire body. Find a vet that has some experience with reptiles...if you don't know of one, just call some vets in the phone book and ask them if they know someone that works on reptiles. Also, there is a forum on Kingsnake for Rainbow boas....a guy named Jeff Clark posts there a lot and has helped countless keepers with their Rainbow problems. He is a gentleman and a scholar. You might try posting this question on that forum and see what kind of response you get.
Good Luck, Chris O

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www.chrisolsonreptiles.com