Oof. What a cliffhanger. That last line was like a punch to the gut.

Stephen's distress at finding the house empty felt very real to me, especially the part where he doesn't realize that he's crying until he hears himself. You have also done a good job adding in all the little background details that show how different "Jon" is from Jon. The scene from the car ride was unsettling, what with "Jon" just sitting there passively, saying nothing (and when he finally does talk, it's that story that he's rehearsed before and used in interviews; how sad!) and then later in the house, when Jon fails to take charge and reassure Stephen... All the little "off" things add up to create this feeling of Stephen descending further into surreality.

This is great so far! Can't wait for the next part.