UPDATE SEPT 2019:
I took this sweet photo of my wife, our son and a nice stranger on the Paris Métro in 2010 and it has since become a meme, viewed and commented on by millions of people. Our family would love to find this gentleman and buy him a coffee. He's been part of our lives for 9 years and we don't even know his name.
Last sighted: August 5th, 2010 in Paris, France. I'm pretty sure we are passing through Gare du Nord on line 4 or 5.