What horrible realization does Elie come to about Rabbi Eliahou and his son, and what is Elie's response to this?

As Elie and his father are catching a bit of rest in a shed on the march/run to
Gleiwitz, Rabbi Eliahu, his mustache ice-laden, finds the father-son pair and inquires about his
own son. Elie notes that this rabbi's words always bring peace, and his countenance has never
lost its innocence.

For three years, the rabbi and his son have endured
everything together: physical abuse, starvation, and camp rotations. And now, the rabbi is in
desperate search of his son, having lost him somewhere along the road. He notes:

We lost sight of one another during the journey. I fell behind a
little, at the rear of the column. I didn't have the strength to run anymore. And my son didn't
notice. That's all I know. Where has he disappeared? Where can I find him?

At first, Elie can't recall seeing his son, and Rabbi Eliahu
leaves without any idea regarding his son's location. And then, suddenly, Elie does recall an
important memory. As they ran, Rabbi Eliahu's son had run alongside Elie for a while. But
the...