Divegirl has got is spot on. I would poster straight away though, put them on telegraph poles and on notice boards at local shops etc.
Not what you want to hear I know, but last week I saw cat that had been knocked down by a car. A few days later I saw a card on a telegraph pole, and phoned the people to tell them what I had seen and gave a description of the cat. It was theirs, but at least they knew what had happened to it.
Also as Divegirl says, check with local vets, and ask people to look in sheds and garages, as many people will have cut the grass recently and may have locked him in a shed by mistake.when putting their mower away.