Talk to your vet about microchipping your cat. The microchip is about the same size as a grain of dry rice that is inserted under the cat's skin, usually in the fatty area between the shoulder blades. On this chip is stored the cat's name, address and basic vet info (vaccinations, etc.) I believe most vet and shelters now have scanners that will pick up the info on the chip, so if kitty is lost he will hopefully returned to you.
Of course this isn't entirely fool proof. Not all shelters have scanners and if he isn't scanned the chip is basically useless. Same may happen if he is taken in by someone who thinks he is a stray. Call your local shelter(s) and vets to see if they have these scanners. If most of them do, I'd say go for it.
I don't know how much it costs to have the cat chipped but you vet should be able to give you a quote.