I just went through a plague of mites. If you visit a pet store or reptile show those little buggers can just crawl up your shoe and hitch a ride, they can come in from anywhere. Even if you sit on a bus next to a snake guy that has mites in his snake room LOL. You could put him on paper towels for a day or two and if he has mites you will see little black specs crawling around.

Personally I would change the the bedding to Reptichip, a coconut husk substrate. It eliminates smell and holds tons of water without being 'wet'. I have it in my whole rack and I'd never go back to anything else unless I had a mite problem again, then back to paper towels for a bit for treatment...