Ok - skip being depressed about how skinny you used to be (weren't we all?!)
He can talk about clothes, toys, cars (if you can find pictures where you can compare the two). Holidays? Maybe as a child you had holidays in the UK and he has gone abroad?
Similarities? Birthdays? Christmas? I'm thinking of our pictures and we have ones of me as a child, in front of a Christmas tree, holding up a present and grinning - and we have almost identical ones of the ds's.
Will try and think of something else but that's off the top of my head.