Put his kitty box outside and go out in the early morning to call for him. A while back my indoor kitty got out and was missing for eleven days. Thankfully, it was during the summer. I found her hiding under a shrub in my side yard very late on the eleventh night. She was too scared to come out to me and could only meow until I extricated her. She lost two pounds which is a lot for a kitty in a short period of time. How she escaped the racoons, foxes, and coyotes I still don't understand. She still makes a run for it from time to time, but then stops in her tracks as she seems to remember, then turns around and runs back in.

Good luck. Keep us posted.