So far I still haven't found him. I've been posting everywhere, calling the shelter every few days. I usually go looking for him in the evening, around seven or so. He's been an outdoor access cat most of his life. I trust him to take care of himself, and he's always come back before.
At the same time, he hasn't been outside for a couple years. Even though he hasn't been acting at all like an old cat (he's thirteen years old) I worry that he overestimated his abilities when he decided to take himself out for a vacation. It's been a month since he escaped now.
I've had him since I was fourteen. I really hoped I'd be able to keep him around for a few more years.