Where does your son and his family live now? Where they got married is not relevant. I need to know that before I can tell you anything, because I am not sure if Texas law applies.
This information is not legal advice and does not form an attorney-client relationship.
He should hire an attorney. The initial question is where. If you can give us more facts about where your son and his wife and child are living (and for how long) we can ponit you in the right direction.
It is also not clear if your son believes the child is his.
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