Does a father's involvement with his money deposited in the bank—which is commingled with usurious funds—render the son's earnings unlawful after profit, and what should be done with the usurious money after repentance and isolating the principal from the interest?

Participating with someone whose money is mixed [with unlawful earnings] is disliked (makrooh), not forbidden (haram), and it does not make the earning unlawful. The questioner should advise his father to repent from dealing with usury (riba). Usurious money must be disposed of by spending it on charitable causes and public welfare. It is not permissible to deposit money in a postal savings account because the postal institution deals with usury.