Contrary to other commenters beliefs I feel you every every right to ask and know the situation about her birth control because your son is a minor and she is engaging in sexual relation with him. Of course you can’t make her do anything but recommending it or simply asking the question is definitely within you right as a parent, who at this point in time would potentially be responsible for any accidental child. And you are most definitely correct that if your son thinks he is mature enough to have sex, he is mature enough to have a simple frank conversation about birth control. If he can’t communicate about birth control he shouldn’t be having sex.