Hello Liz,
Oh my, that is a fungal problem.
Can you review your tank setup with us? What is the humidity in the tank, & what are the temperatures in the tank?
What type of thermometer do you use to measure the temperatures with? A stick on type, a digital probe or a temp gun?
How close is the basking light to him?
Start with getting some betadine, & dilute it down to resemble a VERY weak tea color. This will help clean the skin. Next you can get either tea tree oil or witch hazel. Do not use full strength on the skin, just gently swab it on with a cotton ball or a q-tip.
Then, use some lamisil or monistat cream on the affected area. Do the cream treatments twice daily, & make sure you do a nightly treatment so it stays on overnight for extra time. Then,
bathe him in the morning, & reapply it.
What type of substrate is he on?
Has he been on any medications recently, such as Baytril or Albon for extended periods of time?
Also, you might want to start him on a probiotic such as non dairy soy yogurt, acidophiliz, or benebac to help offset some of the yeast/fungus buildup.
Is he eating very well right now?
Tracie