Put yourself in the shoes of the character for a moment, why would he need to ask such a thing?
As
sillyrobot said, he grew up with her, why would he need to ask for something
he probably already knows, is not like the whole thing was kept secret from him, I doubt in all those years he never asked her such a simple question.
The reason the others have to tell him what they are is that he just meet these girls, if you, as a player, are to get that info, is from a situation where the MC, Ester or even you Mom will need to say that to someone who doesn't know it, like for example Nefari asking what his mother is for the research she is doing, something I think she already knows by the way, or she should have done it from the beginning.