Jack goes still for a moment, just watching him. Taking in the differences that go along with the similarities here, reminding himself that however familiar the face and voice and (let's face it) many of the mannerisms, he knows absolutely nothing about this man.
Apart from which, he's a Time Agent, and how curious he is, he's had it well and truly drilled into him that you don't ask about the important things. The little things, sure, like how much more amazing you are in bed, but the life-changing events? You skirt around those as if they were the plague.
So after a moment, that curiosity gets shoved firmly back down again and he nods, stuffing his hands in his pockets as if to mirror that feeling. "Fair enough," he says mildly, and moves on. "So, who are you anyway?"