Have you had the serious talk with him yet about how his life will be utterly changed, and frankly derailed entirely, if he becomes a father too soon? Does he fully understand what that would involve and that it likely would mean no (or very limited) college options, which means limited job options, and limited income options, and so on -- for the rest of his life?
That sounds like a talk to have right now. As in today. Don't make him mad at you by bad-mouthing the girlfriend; but be factual. You don't want to drive him towards this family by making him feel he has to defend them. He did come to you about this and that is fantastic -- he is communicating with you -- and you need to keep that line of communication open by being non-judgmental. But the idea of a mom emphasizing finding "the one" at 16 and telling the kids to get it on is not only creepy, it's manipulative.
I would also ensure that he and the GF do not, ever, spend much time at the GF's house; be sure they are doing activities when they go out (movie, hike, bowling, dance lessons, whatever!) rather than lots of hanging out time that would lead to other things. I've had friends who are careful that their teens who are dating are out having fun, active experiences and not just hanging around doing nothing at each others' houses.