He is lovely isn`t he? I know I would be tempted to keep him, especially as he is so friendly, and, in my case, if he got on with Bob it would be the clincher, a cat upstairs and a cat downstairs! (only joking)
Seriously, I would do what you are already thinking about, take him to the vet to check for a microchip, plus I would call up Cats Protection and other centres in case someone has been looking for him, put some notices/posters in the local shop(s) and the vets and maybe on some lamposts etc. If you have a local radio station maybe they would give out a found over the air...can`t think of anything else except to maybe ask around if someone has moved house lately, the cat could have made its way back to what he thinks of as home. Good luck, hope everything turns out ok and he finds his home again.

Chris