Put that right out of your head.
It doesn’t even make sense. You didn’t know him long enough or well enough to be having a child with him, when you already have kids.
Having a kid with him doesn’t mean he would have stayed. He still wouldn’t have wanted to be a step parent to your children.
So the outcomes would have been
A - he moved in because you were pregnant, heavily resents you or (at least) your kids or both. He treats his own child very differently and your kids grow up miserable because they live with a step parent who doesn’t like them
B - He moves in realises he doesn’t want to be a step parent and lives back out and you are left co parenting with him. He takes his child 50% of the time or weekends, leaving the others behind reminding them of the difference every times.
Having his own child with you wouldn’t change his mind on wanting to be a step parent to your children. Even I if he does want a child of his own, or forms mean he wants them with someone who already has them.
You are having these thoughts because you were willing to hold on to him regardless of the cost. You would be willing to have a child with a man who doesn’t want to live with your children to keep him. Disrupt your life and your kids lives, impose someone who doesn’t want to live with them on to the kids, have a child and to a man who his telling you he doesn’t want to live with your kids all so you can keep him.
As before, this isn’t about him. You are simply afraid of being alone and imagining all the things you would do if it meant you wouldn’t be alone. You can’t really think that bringing a child into this relationship wouldn’t have solved all your issues and not created new ones.