What is the ruling regarding a father who neither prays nor fasts, nor does he do what Islam commands, despite his knowledge of its affairs, his love for God, and his urging others to do good? And what should we do concerning him?

It is not permissible for the father to abandon prayer, as it is an act of disbelief (kufr). He must beware of the consequences of enjoining good and forbidding evil while he himself violates it, for a severe warning has been issued against those who do so. The children must invite him [to Islam/righteousness] and guide him in the best manner, be patient with him, and frequently supplicate for his guidance.