I agree with Rivlas, has your DH sat down with him and had a 'man to man' talk. I think its really important that your DS is doing it because he wants to and his GF isn't filling his head and making him think he wants something that he doesn't. It sounds to me that his GF hasn't been well educated about having sex, its sad to hear that a 15 yo girl has such a bad reputation.

This situation reminds me of my brother, he went and stayed with his dad and met a girl a year older than him who came across as being a sweet girl but she was really not at all. She played him, made my brother sleep with her and then dumped him the next day breaking his heart. Sometimes it can be just a game and i really don't trust this girl even though i dont know her, a week is too soon to be thinking of sex.