What should I do when my father prevents me from visiting my mother and threatens to expel me from the house if I disobey him?

Visiting the mother is among the greatest acts of righteousness (birr), and it is not permissible for the father to prevent his son from visiting her. If the father orders his son to cut ties with his mother, there is no obedience to him, for there is no obedience to a creature in disobedience to the Creator.

The son must try to convince his father that cutting ties with the mother is one of the major sins. If he is not convinced, then let him visit his mother secretly if possible, or find another house to visit her, because her right takes precedence over the father's right. If he is unable to do so, then the sin is upon his father.