The experience itself, is best addressed to someone inside Chabad, who is part of this program.
If you are asking general advice on how to pick up a better understanding of a)reading hebrew and b)understanding hebrew, perhaps those in the field of education can offer advice.
Regarding the importance of being able to read and understand “biblical hebrew” as you put it, is critical. For many years I volunteered for a group that billed itself as “anti missionary”, their favorite targets and where they have the most success are the ignorant. not just of judaism in general but even those who simply cant read hebrew. They would set up shop with jewish looking symbols and books, printed in hebrew and make claims about their beliefs and base them on pesukim in tanach. With no ability to refute them or even read the source material in the original, many were duped. my point is NOT that you or your son are targets for missionaries, rather, the inability to read and learn something in its original is a HUGE handicap. You are at the mercy of the translator. Like the saying goes, “something is lost in the translation”.