What must a young man do towards his father who mistreated him, slandered his honor, and refused him financial assistance, knowing that the father is asking his son to return to live with him and bear the expenses of banquets despite his difficult financial circumstances?

The right of parents does not fall away due to their injustice or ill-treatment. Allah has commanded accompanying even polytheistic parents with kindness. Al-Bukhari dedicated a chapter to filial piety even if parents are unjust. It was reported from Ibn Abbas that Allah is not pleased with a servant until his parents are pleased with him, even if they have wronged him. You are not obligated to live with your father, nor are you undutiful by living separately with your wife, as long as you maintain connection with him. You are not required to financially support him unless he is in need and you are affluent. Wisdom and tact should be employed to avoid his anger.