Your first mistake was letting your son have a girlfriend so young.
I had a cousin who was doing his girlfriend at that age and it is his single greatest regret.
Honestly, I'd be very clear that you feel he should be abstinent and try to enforce that, as effective or ineffective it may be.
I coach high school baseball and I know very well the mindset of the teenage boy. If you raised them from day 1 to keep their pants on, then they much more likely will. If that wasn't really talked about much, then he's going to think that his pecker is freem to come out whenever he feels like it.
Being raised with a strict rule that dating is to wait until 16, I never did have sex as a teenager because usually by 16 you have better control over raging hormones. In fact, I'm in my mid 20's, single and still a virgin because I was raised that sex is for marriage and to wait, and I am waiting. There have have been several opportunities for me to score but I chose not to.
Tell your kid that if he's horny to spank it before hooking up with his girl.
If he was trying to talk her into it, then she probably doesn't want to and he's in the wrong for being aggressive about getting that poon, which is not love at all. Sex and love are not the same thing.