The question is asked: What was so exceptional about the boy’s answer that Rabbi Yehoshua ben Chananiah declared that he was certain that this child will grow up to be a tremendous Torah scholar?
The Vilna Gaon explains based on the following Gemora in Brochos (4a): It is written: “Ad ya’avor amecha Hashem; ad ya’avor am zu kanisa.” Until your people pass through, Hashem; until this people that You have acquired pass through. Until your people pass through, Hashem is in reference to the first time that the Jewish people entered Eretz Yisroel (in the times of Yehoshua). Until this people that You have acquired pass through is in reference to the second time that the Jewish people entered Eretz Yisroel (in the times of Ezra). From here, the Chachamim have said: The Jewish people deserved that a miracle should be performed for them at the time of Ezra (when they were crossing the Jordan River into Eretz Yisroel) in the same manner that happened to them in the times of Yehoshua. However, due to their sins, they lost this reward.
Accordingly, it can be said that this is what Rabbi Yehoshua was asking: Who gave Yaakov over for spoil and Israel to the plunderers? Although the Jewish people were on a low level when they entered Bavel (as the name Yaakov connotes), but why weren’t they redeemed with a complete redemption (when they were on a higher level, as the name Israel connotes)?
The child answered: It is on account of this (zu); we have sinned against Him. By the second redemption, it is written zu, this; and since we have sinned, we lost the reward that we originally deserved.