What is the correct stance to take when confronting the father about his second marriage, knowing that he abuses the mother and wishes to dispose of her gold, which is in the son's possession?

You must honor your father and be dutiful to him, even if he wronged your mother. It is not your right to object to his marriage to a second wife if he is capable of being just. However, you are not obliged to obey him if he asks you for the key to your mother's money box, and you are not permitted to give it to him without her permission.