It is very difficult for a son to be in such a situation. However, no matter how great sins a father commits, a son has to respect his father even if he is a non-Muslim.

Asma Bint Abi Bakr (may Allah be pleased with her) narrates: "I asked the Prophet (pbuh) about how to treat my mother, who was a polytheist then: "My mother came to my house and wants to talk to me; shall I treat her well?" He said, "Yes, show respect to her. (Bukhari, Hiba 28, Adab 8, Muslim, Zakah 50 (1003); Abu Dawud, Zakah, 34 (1668).

The importance of showing respect to parents and to give them alimony even if they are unbelievers can be understood from the fact that a verse was sent down after the conversation mentioned above and the issue was made certain in the Quran: " Allah forbids you not, with regard to those who fight you not for (your) Faith nor drive you out of your homes, from dealing kindly and justly with them: for Allah loveth those who are just." (al-Mumtahina, 8)

The following verse is clearer regarding showing respect to parents even if they are polytheists: " But if they strive to make the join in worship with Me things of which thou hast no knowledge obey them not; Yet bear them company in this life with justice (and consideration) and follow the way of those who turn to Me (in love)…" (Luqman, 15).