What does Bruno tell Gretel about his imaginary friend (Shmuel) and what does he realize when he is talking about him?

One day it is raining too hard for Bruno to go outside. He is lying on his bed reading when Gretel comes into his room, bored. He accidentally mentions Shmuel to her and quickly covers his mistake by telling her that he is referring to his imaginary friend. In order to sell his lie, he remembers a time when Grandmother had walked in on him while he was going to the bathroom and other embarrassing moments so that his face will turn red. It works, and Gretel begins to make fun of him for having an imaginary friend. Bruno takes the opportunity to talk about Shmuel, now that he has made sure Gretel believes the boy is imaginary.

Bruno tells Gretel about how all of Shmuel's friends with whom he used to play "disappeared without even saying goodbye to him" (158). He goes on and tells her that yesterday, Shmuel told him that his grandfather has been missing for days and that whenever he asks his father about it, his father begins to cry and hugs him. As he describes what Shmuel has told him to Gretel, Bruno begins to feel guilty for not comforting Shmuel when his friend was telling him these things. Instead, he had changed the subject because he had felt uncomfortable. He decides to apologize for his callousness when he sees Shmuel the next day.

oi?