What are the limits of dealing with a tyrannical and stubborn father who mistreats the mother and tries to extort his son to prevent him from helping his mother, and what are this father's rights over his son so that he does not displease Allah, while the son continues to care for his mother and speak the truth?

It is obligatory to be dutiful to one's parent and obey him in what is good and permissible. It is permissible for a son to stand with his wronged mother against his father and prevent him from oppressing her, as this is an act of dutifulness to him. Disobeying one's parent is not permissible under the pretext of his injustice. The husband must treat his wife with kindness and refrain from harming her.