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You are asking two really different things, so I will address both.
If paternity is not established and the man marries the mother, this has no impact on the grandparents having shared custody or guardianship
with the mother. This is solely up to the mother's discretion in the matter and the man has no say even if he marries her unless he adopts the child through the court.
Second, if a paternity test is done and he is found to be the father, then he has rights to stop the guardianship with the grandparents as part of his paternal rights, whether he marries the mother or not.
Thus, the rights of the man depend wholly on whether or not the paternity test comes back showing him to be the biological father.
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