Good job you are vigilant -Keep your eye on him / her. If you feel happy to do so make sure that you leave some food. If he is lost and doesn't know the way home, getting fed at yours may mean he at least stays in the area.

I would call Cats Protection (or your local vet), It maybe he is lost and he maybe chipped. I don't know what the procedure is and whether they come out as I haven't had much experience with CP. But if they did they could scan the poor little lamb and see if he is chipped and may be able to get in touch with the owners.

If he has a collar on, he is likely to have a home and may have wandered away or might be a randy little sod, shouting for a mate

Also people often advertise on Gumtree about lost cats so have a look on that. You could start an ad (it's free) Describing him and the area he is in. The only thing I would say about Gumtree is keep your details of contact private as there are some real nutters who use that site, as well as some really nice, valid people!

keep your eye out for him and let us know.