Do yourself a favor and adopt a pair. Bonded pairs are the best but not required. Cats tend to keep each other company and tend to adjust more easily. And, frankly, taking care of two isn't much harder than one apart from an extra litter box.

There's pawboost and a few Philly Lost and Found groups, try them if you haven't already.

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Otherwise, hope you can catch him. Get him checked for FeLV/FIV if you have cats touch him.

Put his litter box out and some clothes. Also post to pawboost and the various Philly area FB pages. Flyers are always good to post around, too.