Ah nachas. Great story baal habooze. I found some answers on aish dot com on Rav Asher Alports torah page that you can share with your son.
The Torah forbids the consumption of orlah, the fruits produced by a tree for the first three years (Leviticus 19:23). The Talmud (Shabbos 33b) relates that when Rebbi Shimon bar Yochai was forced to flee to a cave to save his life, a carob tree miraculously sprouted there to provide him sustenance. How was he permitted to eat the fruits, which are considered orlah?
Rabbi Yissochar Dov of Belz (Imrei Daas) cites the Talmud (Yerushalmi Orlah 1:1), which rules that if a tree grows in a place which isn’t designed for human settlement, which was the case with the cave of Rebbi Shimon, it is exempt from the laws of orlah.
Rabbi Chaim Kanievsky (Derech Emunah Hilchos Maaser Sheini 10:6) notes that Maimonides rules that if a tree grows on its own in a public area, such that its fruits are ownerless and available to all, the laws of orlah do not apply to it.
The M’rafsin Igri gives a few answers. First, a tree which grows miraculously is exempt from all laws governing fruits, such as orlah and ma’aser. Second, the tree may have already been planted elsewhere for more than three years, and if it was transferred to the cave together with its roots, its fruits would be immediately permissible. Alternatively, Rebbi Shimon may have eaten the carob fruits while they were still small and not yet legally classified as fruits which are forbidden as orlah.