Putting signs up and an ad in the paper, and calling local vets and animal control to see if someone has reported a missing cat like this. And perhaps he has a microchip? A vet could determine this.

Please don't just let him out, Gary! The weather and the traffic and the, uh, ..not-so-nice people and animals in the area would be too dangerous for him. If he hasn't gone home before, he won't now. He either doesn't have another home or he was driven out from it.