The questioner asks: Indeed he (i.e. this person) had a disagreement with his father about some wealth and this led him to hate his father, shouted at him and refused to sit with him; so is he sinful for boycotting his father, who is now very old and ill?
Answer: What this son has done to his father is Uqooq (disobedience, disrespectfulness and undutifulnes to parents) and it is a major sin among the major sins, and we see Allaah’s protection from it. This son is (exposes) himself to a great danger and it is obligatory upon him to repent to Allaah. He should return to his father in order to make him happy, seek his pardon, seek to please him and honour him. Allaah (The Most High ) said:
وَاعْبُدُوا اللَّهَ وَلَا تُشْرِكُوا بِهِ شَيْئًا ۖ وَبِالْوَالِدَيْنِ إِحْسَانًا
Worship Allah and join none with Him in worship, and do good to parents..[4:36]
And Allaah (Glorified be He and free is He from all imperfections) said:
وَإِن جَاهَدَاكَ عَلَىٰ أَن تُشْرِكَ بِي مَا لَيْسَ لَكَ بِهِ عِلْمٌ فَلَا تُطِعْهُمَا ۖ وَصَاحِبْهُمَا فِي الدُّنْيَا مَعْرُوفًا
But if they (both) strive with you to make you join in worship with Me others that of which you have no knowledge, then obey them not, but behave with them in the world kindly…[31:15]
Good behaviour with parents is required of a person towards non-Muslim parents, then how about if a person’s a parent is a Muslim!
[Fadaa’il Al-A’maal pages 16-17’ by Shaikh Muhammad Bin Abdillaah As-Subayyil (rahimahullaah). Abridged and slightly paraphrased]