Most likely you are at the leading edge of scale rot. At this point it can be easily treated with three steps 1) switch to paper substrate temporarily, 2) apply plain Neosporin to affected scales daily, and 3) wait out 1-2 sheds for scale rot to disappear before quitting 1 and 2. You've probably either over-misted the substrate or he's been soaking and then getting it wet and staying in it.