What is the ruling of Islamic law concerning a father's ill-treatment of his son and defaming him, and what should the son do in such a situation?

Allah has made it obligatory to honor one's parents and obey them in what is right, placing their right after His own. Therefore, one must treat them kindly, even if they are abusive or polytheists. It is not permissible for a father to abuse his child, and if he does, the son must not return his abuse.