Wow you guys aren't going to believe this... I started looking again upstairs even though I had pretty much given up hope of him being there after I posted this... I turned on the lights in the upstairs bathroom, and he scattered back into the TINY space between the cabinet and the wall where he'd been hiding. I tried to get a hold of his tail but he wedged himself in pretty good and started hissing at me. I put one of his hides right outside of the crack and turned off the lights, so hopefully he'll decide to go into it.
Thanks so much! I wouldn't have even kept looking in that area!