I'm assuming that your nineteen year old son is still living with you, because otherwise it's entirely irrelevant.
He could certainly try making that argument in court and seeing how far it gets him. If the girlfriend is "decent" (she's not a druggie or a skank), it probably won't get him very far, unless you're in a very conservative state. Many states no longer even allow "morality clauses" saying that you can't have a boyfriend spend the night while the child is present, and as long as your son's girlfriend isn't a "negative influence" then it's not likely to have any effect.
If paternity has not been established, then you would have to do that first. Once paternity is established, then you have to right to petition for custody.
Eminem has custody of his daughter and also his niece- his ex-wife's sister's daughter.
no, My daughter had a baby at fifteen and I Had custody until she turned 18.
no
no no
No. His wife Priya Narang had Primary custody of their daughter.
Through some kind of family inclusion of the girlfriend, the mother of the boy can gain custody. An option could be adoption.
No, you are an adult at 18.
yes he should do
file for custody
For 1) are you in custody of your daughter? If you are in full custody you could take her to court for kidnapping is your EX mother in law in custody of your daughter? Do you have joint custody? If your ex mother in law has no custody over your daughter you could take her to court, she is not authorized to take care of your child.
see related question