He said to his son: If you do not stop fighting, I shall divorce your mother

Praise be to Allah.

If the husband says to his son, “If you do not stop fighting I shall say to your mother: You are divorced,” that does not count as a divorce as such, because it is a threat to issue a divorce if the son persists in fighting. Then if the husband wants to issue the divorce he may, and if he does not want to, he may refrain. 

Secondly: 

If the divorce is issued in a moment of anger, it is subject to further discussion. This has been discussed in the answer to question number 45174. 

The husband should fear Allah and avoid uttering the word of divorce in when angry or otherwise, unless he actually intends to issue a divorce, after having thought long and hard about the consequences. Divorce was not ordained to let off steam in the case of anger; rather it was ordained to end a marriage relationship in cases of necessity. 

May Allah help us all to do that which He loves and pleases Him. 

And Allah knows best.