For reasons that are not yet clear to me, this particular post and its responses have been niggling at me all day and I felt I had to come back and say more.
I absolutely get that caution needs to be taken in such an instance and for me, from your post the issues I see are how stable this woman is and how keen your son actually is (is he keen, or has she just imposed herself on him for example?) These are not questions I can possibly know the answers to, but I suspect deep down, you've got a pretty good inkling whether this is a good thing or not (and I don't mean to imply that it is one or the other - none of us can know. You know your son, you know this girl, you know the full story).
What concerns me about the 'you have to put your foot down' approach to this is that if your son really is very keen on this girl you risk the evidently very close relationship you have. This isn't underage drinking, this isn't illegal activity, this isn't not getting your school work done - all instances where I see it as an appropriate place as a parent for me to say 'not good enough' and follow up with whatever consequences are appropriate. This is something much more important - your son's feelings, his emotions - maybe his first love. Much more fragile. You can't just ask him to switch that off and get over it (which I know you're not saying you'd do - this is more in reference to some of the subsequent comments). If someone had done that to me it wouldn't have stopped me seeing the older man I was in a relationship with - instead I would have just done it behind my family's back and been in a much more dangerous position had it all fallen apart.
I'm not saying tell him to go for it and forget the consequences. I'm just saying that this needs much more tact. Your son might not have any idea about how he feels, he might not be as keen as this girl and need some guidance on what to do - or might feel the same way as her and seems to be taking things fairly slowly, which he should be commended for.
If you do have some really strong concerns then you need to intervene, but I'd think carefully about the best way to handle it (and of course, as Austin mentioned, both he and the girl need to be aware of any legal consequences).
I absolutely agree with Grinity (as I so often find that I do!) Talk to the girl, find out more about anything you're worried about. Talk to your son. And listen to your gut.
