Aspen bedding is dehydrating for snakes...so it's good for some kinds of snakes, but not preferred by BPs. Or maybe he's just bored & soaking is a new experience? I'd still be checking for mites: how long have you had him? do you have any other snakes? new ones? held snakes in a pet store or at a
friend's house & then held your snake without showering & changing clothes? Mites sometimes hitch a ride on humans, & then proliferate exponentially
on your snake...it only takes one mite to start a colony & kill an unprotected pet, so check thoroughly...up close & personal...they're hard to see.