Very nice looking "Yardus"-Lampropeltis you have there.
You can try plugging in a (human) heat pad, put that on the floor over night for a few days and even bait it with stunk up paper towel from under the next time you thaw a rodent for feed.
(Or rodent bedding in you raise your own)
When he comes out to hunt (or look for his way out) he might stay under the hide on the heat.
Take more pictures of him before you let him go.
Good luck.
Chris