Well if it's only the bedding & not mites, he seems to have a "solution" for the aspen- it's possible he finds it's irritating to his skin too? Snakes can probably be allergic to
things, though it's not common (or else goes undiagnosed), so watch for signs of skin irritation (rash?) while you're searching for mites...
A BP "that loves to play in water" usually is soaking due to discomfort from mites, dehydration, or something irritating their skin.
I've never heard of aspen irritating a BPs scales?
Also exactly, unless you know its mites don't treat anything!!