What is the best way to advise a father who watches movies and plays that contain forbidden scenes?

You must forbid your father from watching movies and TV series with wisdom and good counsel, reminding him of Allah Almighty and the obligation to lower the gaze, citing His (Allah's) saying: "Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do." [An-Nur: 30]. If he complies, then praise be to Allah. If he does not comply, then you have fulfilled your duty.