Once he warms up to about room temperature get him a heat mat to warm him more. Since he was in a rock crevice he was probably trying to hibernatebut since he's warming up I wouldn't risk releasing him again. I would say keep him until the springish if you can. Or you could slowly drop the temperature in his tub so he hibernates again. Then release him. Maybe. I'm really not sure what to do in this situation. But that's what i'm thinking..
*edit *Oh, and if you decide to keep him you'll need to feed him an appropriately sized pinky mouse or feeder fish depending on how big he is.
Keep him in a smallish container with air holes so he doesn't get stressed by a large size with a heat mat to keep him warm. Add a substrate (paper towel, aspen, newspaper), a waterbowl, and a few hides. (you can take a container and cut a doorway in it and flip it upside down)
He's might be a ring necked snake, worm snake, crayfish snake, or even cornsnake (but don't take my word for it, i'm terrible at classifying snakes). If you want you can google virginian snakes to see which one he looks like.