Well, I have a couple of ideas, based on my own personal experience: I have been happily married for 11 years, but before then I was engaged, then left that guy at the last minute because I had feelings for my former boyfriend, who used to keep me hanging, but who I ended up ending all contacts with when I finally realized HE was never going to make up his mind about me.
Funnily enough, after I closed up my contact with this ex-boyfriend, I met my current husband who is the most loving and caring and great person ever. I believe that if I did not end my indecision about my ex, I would have never been ready for a good new chapter in my life. And the truth is that the guy I was engaged to before was not right for me, even though he was a great guy, and I am happy we did not marry because I would have had doubts later, no question about it.
So, based on my experience, here it is: if she left him at the last minute for an ex, she does not love your son enough to make him happy. If she is that unstable at the beginning, when all you see is the positive and you are really up in the clouds, then it will not work and your son will have to fight on and off all the time to keep her love.
He does not deserve this at all. But unless he decides to end it, she will most likely keep him hoping for a while, like keeping her options open in case the other guys does not work out.
I can tell you all this from the bottom of my heart. I was in her position and thinking back I was not a very good person - not on purpose, but I wasn't as honest as I became later. And now that I love my husband to death even after 11 years, I can tell you that doubts at the early stages of a relationship will only get worse.
And my advice to you on how to get this over, if you are really up to it and if you do not think it's off the rails, is to call the girl yourself and have a chat with her one-on-one without your son there. Tell her to decide one way or the other and to stop leading them on (or whatever she is doing that is not ok). It's either yes or no, not a maybe, and she needs to decide now and tell you what she is giong to tell your son. You definitely love your son more than she loves him (my opinion only), so you have every right to do something to ensure he will be happy.
Hope this helps a little:)