If he wants to do it, then trying to stop him is probably not the answer. Filling his bedroom with condoms is going to be the way to go. I agree with Jennifer that you should nurture the relationship. Don't fight it, no matter what you've heard about her, or what you think. Support it - always offer for them to stay at yours, so that if they're doing it, you know it's nearby. And really really stress to him to make it safe.