It sounds as though you are in shock over this situation and I'm sorry there have been very harsh comments in some posts.
I get the impression that, if your DS and his GF had been in a stable relationship for a long time preferably with a home of their own and had both decided to have a baby, you would feel differently and be looking forward to getting to know your GC.
That's not the position though and you're probably frightened that there is so much potential for people, including your as yet unborn GC, getting badly hurt through this situation. The strength of your reaction may be connected to that fear.
Working while looking after a young autistic child and considering having another child is a lot to deal with already.
You and your DH must try to be on the same page and think about your future family life - your son, with or without his girlfriend and baby will presumably not still be staying with you in a couple of years time.
Your husband may then regret not having another child of your own - especially if your son and girlfriend should split up and you end up hardly seeing your GC anyway.
In the meantime, you have to decide what to do.
I doubt having them moving between your house and her mum's house is sustainable or good for anyone - other than for a very short time. They need to sort out their own accommodation fairly quickly. Equally, you need to tell them what you can and cannot do to support them: whether you would put them up at all and, if so, for how long; whether you might be in a position to help financially either short or long term; reassure your son that you will always love him and will love your GC.
Your son is probably feeling lost and panicky given the circumstances and needs to know you love him and will help him within whatever limits you choose to set.
If you and your husband can't fully agree, perhaps it would be worth considering seeing a counsellor on your own to help you with the impact of all of this.
It is perfectly possible that your son and his girlfriend settle down, get their own home and you have a great relationship with them. I would try not to close the door on a relationship with his girlfriend or your grandchild but also avoid feeling pressured to change your plans and family life to solve their problems - they are adults even if they're still young. I hope things work out far better than you could imagine.