Count your lucky stars that in 4 yrs that heat rock never burned him...heat rocks are the devil lol! Definitely get a UTH ASAP...Amazon.com has 10 gal ones for cheaper than any pet retail store.

Once his skin "clears up" again and he begins to slough around the nose, soak him for 30 minutes in warm water (make sure to have a water dish flipped upside down in the tub so he can rest). Then do what Zack said and just run him through your hands and the skin will come off like a glove.

Personally, I've found it hard to keep humidity in larger tanks unless half of the screen lid covered with glass.