Let's please discuss this for a second-
He's "seeing" things from the future that haven't happened. Mikasa and Armin are presently just a twinkle in their parents' eyes.
This guy's name is Eren Kruger.
He looks identical to Eren- compare the pictures of young Kruger and young Eren. They look so, so similar.
However we know that Kruger is not related to Grisha, and it's impossible for him to be related to Carla, so either it's an uncanny coincidence that he looks like Eren or he is Eren. We know that there's some weird futuristic time travel-y stuff going on- "To you, 2000 years from now" and "See you later, Eren" and this whole "Paths" thing.