Has he attempted to wipe it off himself? Rubbing it up against the substrate or walls or anything he can find? If he has, and it's not coming off, you might check to make sure it's actually feces and not something else.
Then, get one of those Kritter Keepers, fill it with just enough warm (just above lukewarm) water to reach his belly, and then set him in there for about 10 minutes. After 10 minutes, take a soft cloth and very gently wipe the feces off.
And, not to lecture or start a war, but this the teacher & mother coming out in me, I guess - but please refrain from using Profanity. There are MANY synonyms for a butt.
~~Dusty Windom
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So Many Alleles, So Little Time...! 


@
~~The Gecko Barn~~