Rule 1:  The welfare of a child must come first.  Speak with his legal guardian(s) about Poser in general and how it would relate to your brother's current and future artistic trends.  Unless there is some hidden agenda on your brother's part, the odds are good that what he is drawing now is what he will start to try in Poser.  Naturally, if he has the full library, he will want to see what it has and, if he draws humans, he pretty much needs to start with a nude Poser figure in order to dress and pose it.  If and how he dresses and poses it would again probably start with what he is drawing, within the limits of the program.

Unless he keeps any naughty art hidden so you have know idea he drew any or secretly draws such things for his friends, I doubt having a nude Poser base required for most anything else will really matter.