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I don't think the answer is expecting him to choose between you and the expectant mother. He will almost certainly choose her at this point, and then you've lost not only a grandchild, but your son. Let him know he is always welcome in your home, and you hope to have a chance to meet your grandchild. Then, drop it. Say nothing about the girl.
It won't be easy on you, but chances are good they won't make it long as a couple based on what you've said. If he's there for the birth, and as long as the relationship lasts afterwards, it will show the court he wants to be involved in his child's life.