When Eliezer realizes that the Rabbi's son has indeed abandoned him, it is reflective of the lessons that Wiesel wants to instruct the reader about during the Holocaust. One particular lesson is symbolic. The Rabbi, the symbol of supreme religious identity in the Jewish community, is confused and has been abandoned. This becomes representative of how Wiesel constructs the notion of the divine in the narrative. From the time of Moshe the Beadle to Akiba Drumer to Eliezer's own rejection of the divine, there has been a distinct reality woven through the narrative as to how the divine has abandoned its followers and how its followers have abandoned the divine. This becomes one of the fundamental reasons why Wiesel includes the abandonment of the Rabbi.
On another level, the inclusion of this event helps to highlight what Wiesel sees as one of the most basic and sad lessons of the Holocaust. Outside of the massive scale of mass murder and state sponsored horror, the Holocaust is a lesson in what happens when individuals sever the bonds between one another. This alienation and estrangement becomes one of the lessons of the Holocaust. Wiesel is seeking to show what happens when people abandon one another. There is a certain terror in Eliezer's realization that the Rabbi's son abandoned his father because he did not want to be bothered with him. This terror is only enhanced when Eliezer does a similar thing to his own father. It is in this reality in which the Holocaust's true horror is evident.