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.