"The girlfriend ... does everything in her power to stop any relationship."
If true then your son has made a poor choice and now made a baby with her is stuck with her. But his choice. It's down to him now. Or maybe his choice is to prefer her family...
I would wait until the baby arrives and your son becomes a dad. He'll see things differently. Then would be a time to talk with him about your place in his future.
How to Keep Living at Home Longer



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