Tass, probably the best thing you can do here is remember you're the grown-up and the mother. That's often the best strategy for dealing with the things our 17 yos throw at us, and it will probably help here, too.
So, firstly, detach. You'll deal with it all much better if you can separate out your emotions and then be calm with your son.
Crucially, look after yourself. Acknowledge your own feelings and give them some space. You probably won't feel the same as your son, and you'll be better able to let him feel whatever he feels, if you are recognising your own feelings. If you're upset and angry, that's not surprising. Find a friend to weep and rant with, or if not a friend, a counsellor or parent support phone line. If you're unhappy or stressed, do some nice, fun things for yourself - relax or laugh or have fun - and you'll then have more energy for supporting your son with what he needs.
Be practical. Whatever anyone feels, certain things have to be done. And doing things might help you feel less powerless, too.
Work out what you need and want in this situation. What's your 'bottom line'? Are you prepared to support the child? Adopt it? Have them live with you? Etc... I'm not suggesting you should - or shouldn't - do any of these. But if you work out what you do and don't want yourself, you won't get pulled into situations you'll resent, and you'll be better able to support your son to do the same.
Bear in mind you might be the only adult in this situation. The woman, if you've judged her accurately, doesn't sound very mature or happy. That doesn't mean 'taking over': adults respect other people's views and autonomy... But it may be that they can't do that without help. You might have a useful role 'mediating' and keeping conversations civil, focused and respectful.
Be positive. It might not be a disaster. Most situations, after all, are shaped by the way people behave in them. If you can support your son to behave well, it might all turn out well. :) fwiw, I have a good friend who is just 32 and who has kids aged 18 and 16 (yes). It hasn't all been easy, of course, but she has brought them up well, gained a degree while she has been doing it, and her eldest is off to uni herself in a couple of weeks. Unplanned babies can be the start of life, not the end of it. :)
Good luck!