Agree with Hec completely.
His motives for seeing her or her motives for seeing him are neither here nor there. Even if you knew them, it would only be another thing to be in disagreement about, ie, the motives are never going to be good enough in your eyes because you have your opinion and that's it.
I think you will always see him as your little boy even when he is 45
Stand by him, be supportive, invite her over and be genuinely nice and open about the whole thing. If it's going to last then at least you will have a good relationship, all of you and either way your son will realise he has a really cool mum who supports him and lets him live his life. It's not what you wanted for him, but, it's what he wants for himself that counts. You never know, maybe this girl will be lovely and her kids may be an absolute joy?
I remember being 17 and smugly thought I knew EVERYTHING about life, only now can I look back and realise that I actually knew nothing
Don't start on the path of being a nasty MIL with all of the misery that accompanies it for all parties. I honestly think it won't last but let him decide that on his own with the knowledge that are you there for him and that you are fab because you supported him