Well considering mites are nocturnal, he may have had them for quite some time before you noticed. Engored female mites are most often what we see, as anything younger than an adult mite is very very hard to see unless you are purposely looking over the entire snake. When the are full of blood is when we see them, and even then, they don't make most of their moves till night time. He may be covered in them at night, but you haven't seen it till now.

I'd treat everyones enclosure and the surrounding area with PAM. Put the bag of aspen in the freezer or toss it entirely.