I would make a few business-card-sized cards. Put one in a jacket pocket, one in a pants pocket, and one in his backpack. Hide one in his shoe if you're really worried.
For more visibility, you could also give your son a lanyard with an ID card pocket and stick a card in that.
I would laminate the cards to make them durable against water and folding/tearing.
I would write a short notice on the card, in the country's local language plus English, something like:
Hello, my name is Matthew and I'm lost. Please contact my parents Eric and Melissa on their cell phones +1-123-555-1234 or notify the police or security staff.
I would not add too many details, but provide enough to make it quick to get in touch with the parents or other guardians. You could also make special-purpose cards if you're often going to a particular mall or playground, or to mention other guardians than the parents (e.g. when on a trip with grandma).
I would not do much in terms of pretty layout (as in a business card). Just make the text as large as possible, to be easy to read even for youngsters and people who use reading glasses.
I don't see a reason to add a photo of the child, but you could do that if you want. It might be a good idea to put a photo of the parents on the reverse side, so that the finder knows who to look for.