I am a bit confused by this situation -- I'm sure it is much clearer from your side!
An unschooled child, who is essentially in charge of his own education/quest for knowledge, is asking someone with knowledge for more information .... and you are looking for ways to avoid giving it to him.
He doesn't sound fragile, and has in fact already shared his opinion this his style of learning is better. The boy may in fact be thinking ahead and wondering if he wants to study math, or music, or history (or whatever!) in order to further his future goals. While I would personally advise giving the information in a neutral manner (no hint of which style is "best"), I can't imagine he would be harmed by knowing why you value a classical education, and he may be helped if it motivates him to pursue more knowledge on his own.
I think the bigger question is whether it would harm your relationship with his mother, as parents can be very defensive about their choices.