What is the ruling regarding a man who swore to his son that he would shave his beard, otherwise he would divorce his mother?

It is forbidden for a man to shave his beard. It is not permissible for a father to order his son to shave it, nor is it permissible for the son to obey him in this matter, because obedience to Allah and His Messenger takes precedence over obedience to anyone else.

If the father swears an oath of divorce upon his son to shave his beard: If he only intended to threaten, then the father must offer expiation for a broken oath (feeding ten poor persons, or clothing them, or freeing a slave; if he cannot find any, then fasting for three days), and the son continues to grow his beard. If he intended for the divorce to take effect, and the son refused to shave, then one divorce occurs. If it becomes clear to the son that his father intended for the divorce to take effect, and this divorce would be the third, leading to the irreversible separation of his mother, then he may shave his beard to avert this great mischief, and then he should return to growing it.