I would think that legally, if your son is expected to pay child support - then he would have visitation rights to his child as well, just as in a divorce situation.
And most court systems are recognizing the very real importance of having a father in a child's life. Don't just lie down and accept that you won't see your grandchild.
On a more personal side, consider the fact that the young lady was being reasonable until she moved back in with her parents. It is possible that she is being pushed into this position.
Many parents upon learning that their unwed daughter has become pregnant take the stance that it is all the boy's fault. The idea of "two to tango" kind of goes out the window when it comes to responsibility (or blame rather). Deep down they are hurt and furious with their daughter, but in trying to be supportive of her they turn that anger onto the boyfriend. The fact that your son is an ex-boyfriend makes him an even bigger target for their anger. You said your son was 24, but you never mentioned the age of the young lady. This might also have something to do with it.
The best thing I can think to do (and I have never been in this situation personally, so I'm just kind of going on instinct here) is for you and your son to try and stay in touch with the family, offer support to the young lady, and only take the hard line when it becomes absolutely necessary. I realize this is not far off, but try to be as kind as long as possible.