He swore to divorce his wife in order to force his son to shave his beard
Question

                He insisted with an oath that his son shave his beard, otherwise he would divorce his mother. What is the ruling on that?.
Praise be to Allah.Firstly: 
It is haraam 
    for a man to shave his beard, because of the ahaadeeth which clearly enjoin 
    letting the beard grow, such as the hadeeth in which the Prophet  (peace 
    and blessings of Allaah be upon him) said: “Trim the moustache and let the 
    beard grow.” Narrated by al-Bukhaari (5554) and Muslim (259). And he  
    (peace and blessings of Allaah be upon him) said: “Be different from the 
    mushrikeen; let the beard grow and trim the moustache.” Narrated by 
    al-Bukhaari (5553). 
And he  
    (peace and blessings of Allaah be upon him) said: “Trim the moustache and 
    let the beard grow; be different from the Magians.” Narrated by Muslim 
    (260). 
Al-‘Allaamah 
    Ibn Muflih (may Allaah have mercy on him) said: Ibn Hazm stated that there 
    was consensus that cutting the moustache and letting the beard grow is 
    obligatory. End quote from al-Furoo’ (1/130). 
The Standing 
    Committee for Issuing Fatwas was asked: I am a young Muslim man and I want 
    to let my beard grow, but my father is strongly opposed to that. Should I 
    let my beard grow or obey my father? 
They 
    replied: Shaving the beard is haraam and it is not permissible to do it in 
    order to obey a father or a boss, because obedience is only required with 
    regard to that which is right and proper. The Prophet  (peace and 
    blessings of Allaah be upon him) said: “There is no obedience to any created 
    being if it involves disobedience towards the Creator.” End quote from 
    Fataawa al-Lajnah al-Daa’imah (5/146). 
Based on 
    this, it is not permissible for the father to tell his son to shave it, and 
    it is not permissible for the son to obey him in that.  
But the son 
    must honour his father and obey him in other matters, and he should explain 
    to him that obedience towards Allaah and His Messenger takes precedence over 
    obeying anyone else. 
Secondly: 
If the 
    father swore to his son that he would divorce his wife if he shaved his 
    beard, then his intention should be examined. If he meant to threaten him 
    and alarm him, and urge the son to obey him, then this comes under the 
    ruling on oaths, according to one of the two scholarly opinions. This is 
    what a number of contemporary scholars have based their fatwas on.  So the 
    son should continue to let his beard grow, and the father should offer 
    expiation for breaking his oath, which is freeing a slave, or feeding or 
    clothing ten poor persons; if he cannot do that then he must fast for three 
    days. 
If his 
    intention was to issue a divorce if his son did not shave his beard, then if 
    his son continues to let his beard grow, one divorce (talaaq) has taken 
    place. The sin here is not on the part of the son, rather it is on the part 
    of the father who introduced the issue of divorce into this matter. The 
    consequences of divorce will affect him and his whole family, so let him 
    fear Allaah, may He be exalted, and adhere to His sacred limits, and not 
    combine a sin with harm to himself and his family. 
Can the son 
    shave his beard once, so as to avoid the divorce of his mother being counted 
    or not? 
It seems, 
    and Allaah knows best, that if he believes that his father intended that a 
    divorce should take place, and not to threaten or alarm him, and this is a 
    third divorce, which would lead to irrevocable divorce for the mother, then 
    he may shave his beard in order to ward off the greater evil. If he shaves 
    his beard, this will cancel out the oath of divorce, then he can let his 
    beard grow again. In that case the father should know that he cannot be 
    obeyed in this matter, and if he swears to divorce her again, he will not 
    harm anyone but himself. 
May Allaah 
    help us all to do that which He loves and which pleases Him. 
And Allaah 
    knows best.