Well, he was human and looking for someone, rather than alien and looking for someone. That made the Doctor pay attention when he spoke. He was not, even in happier times, waltzing around London with Rose, heavily accustomed to having strangers walk up to him and ask for help. He supposed most people were not, though, and that made him feel better.
The Doctor looked at the picture, recognizing Dean instantly, and also, for that matter, Sam. He'd not directly met Sam, but Sam's timeline was full of fixed points, too. The Doctor'd been very close by for some of them, just to watch what'd happened.
The picture proved some kind of relationship to Dean, since in the picture, Dean's arm was around this man.
That made this easier.
"Us? Who's us? Because the us that's me's heard from him," the Doctor said. He smiled blankly as some people--likely not really people-- bustled past on the street, down toward the shops.