They do absorb some water while soaking, and they do need extra hydration both for digesting and shedding, so maybe he just senses that he needs this?
Or maybe he's like the little kid that will only drink thru a straw...have you tried giving him a colorful straw? ;)
On the other hand, maybe he's diabetic? if there IS such a thing??? (I don't see why not...or hyperglycemic?)
http://www.anapsid.org/diabetes.html