Presume that the Mother does not say maariv and that no other male is available to make kiddush on Friday night. Does a young teenager have to be inspected to see if 2 siman hairs are in place? Is there any occasion for which an inspection for 2 simanim hairs is required?
It would be required only in a case that the young teenager is going to be motzei an adult for something that is a d’oraysa. For example; if his mother or father didn’t daven maariv. Another case would be if he is going to bentch and include an adult who has a d’orayso obligation to bentch. (He may lead the zimun or daven as a chazzan as these are d’rabonon, and being bar mitzvah is enough for this). If there is no male to make the Kiddush Friday night, the woman can say it, herself, or even say it along with the son.
In practice though, there isn’t usually a need to put him through the unpleasantness of an inspection, and he just isn’t motzei others for such things, until we can tell by the growth of other hair or other signs of physical maturity.
M:A 198-7, Rema O:CH 55-5, and M:B 31, .