My idea is a do it yourself thing... You can get card stock from the store (heavy cardboard type paper) and print one yourself if you have a computer. Make it up yourself... This way you can put the information that you want on it. You can even inclue a pic of you and Dad if you want. That way people know who to return him to.
You can buy contact paper or laminating sheets from Wal-mart so that it lasts a little longer. If he doesn't resist, you can punch a hole through it and have him wear it around his neck. If he doesn't like that, maybe make a small ID badgle and try pinning it to his shirt (front or back depending on what he will let you do).
I wish you all the best. My daughter is 3. Fortunately for us she talks to EVERYONE. I know I would be devastated if I ever lost her!