This place you must go to as they are not helpful over the phone. Give them a flyer with his photo. Fill out a lost animal report and request to see all strays. Do that daily. I know it sounds nuts but if they do collect him, it's the easiest way to ensure you get him back. Also ask them if they have picked up any injured or dead cats meeting his description. Sorry I know that's awful to say but it's important to keep drilling them politely with questions.
Contact all vet clinics in your area as well in case someone brought in an injured cat meeting your description.