You sound really involved in your DS's grown up decisions - so it sounds like the two of you are close. Your own DS's father was useless, you're disabled in a way that makes travelling difficult... if you hadn't had such a great DM support you, life would have been even harder. Your DM was your support. Not the man who fathered your child, but your DM - she's the one you could count on.
And you don't want this woman to move closer to her DM for support. Back when you were pregnant, how would you have felt about being given similar orders?
I appreciate your DS wants to step up. But life has a habit of repeating itself. I know so many men who have ended up doing exactly the same as their fathers - whether in an admirable or despicable way - and I'm not sure I'd want to rely on your DS either in this situation.
You say all their friends are here, but also imply that your DS and this woman casually sleeping together blew up their friendship group. So, not a lot of non-familial support for her, and a lot of hurt that is going to linger for this child's life. Being around different friends - and new friends - may be less emotionally difficult.
This woman is moving. She's being smart about this. Even if you could, stopping her would not be in her or the baby's best interests. You've clearly sacrificed for your own child, so as much as this reality is not what you wanted, part of you must understand and respect what she's doing.
You're also assuming she wants to move. Honestly, from all you've said, I'm not sure she has many options available to her - she may well be moving because she feels she has to.
Your DS needs to look at what is in the art of the possible.
Can he relocate? If this woman has managed to get a change of office, it's unlikely she's moving to a tiny village. If it's a bigger city, there's hope of finding work there.
Can he start saving for this child? Whatever relationship he does or doesn't have with the child, being in a position to give the child a lump sum when they finish school is always going to be useful.
He came up with a very detailed fantasy over how he could help with changing his work hours, doing groceries... but what would she actually like?
Can he ask her what she would feel most supported by?
If she doesn't know, could she agree to let him ask that same question again in X months' time?
He doesn't want to be his father. Sure, but that's making this about him. Rather than brainstorm ways to not be like his father, what does she think he can do to be a good father as himself to this child?