My father is not convinced by the benefits of mail, so what should I do?

The son must advise his father gently and wisely regarding the prohibition of usury. If the father does not respond, there is no sin upon the son. As for dealing with the father's money, which is mixed with both lawful and unlawful earnings, it is permissible to eat from it, accept gifts, borrow from it, and so forth, as long as the dealing is not with the usurious interest itself. This is only permissible in cases of urgent necessity, such as the father's maintenance, and the necessity is assessed according to its extent. If the son becomes self-sufficient and no longer needs this unlawful money, then it becomes forbidden for him to deal with it.