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... There are also some keepers
that swear their snakes caught mites FROM a bag of aspen (or from other products) brought home from a pet store where snakes were kept that had mites.