I had the same conversation 5 years ago, and they ended up doing it anyways. I still remember the text I saw on his phone:
"You're lucky your so darn cute that I just have to fall into whatever you want me to do."
This is the mind of a lovestruck horny boy.
I called him on it, and asked him specifically if he would have sex with her if she wanted. Of course he said no, but they did anyways months later, and did it often.
I hope that your son was sincere, and since I don't know him nor did I hear his tone in response, I will say that you need to be the one to determine whether or not he was sincere. Ignore any impulse that tells you to deny your son would lie to you in an important conversation, and be objective. If he really was sincere, then you have made progress.
The next step is to talk with her parents. Sooner rather than later.
Also, tonight, kiss your son while he's in bed and tell him you love him.