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

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.