Should he do Hajj on behalf of his grandmother even though his father does not approve?
Question

                Is obedience to parents obligatory in all cases? If they tell me not to do a naafil act of worship such as a voluntary fast or a naafil prayer, do I have to obey them? I resolved to perform Hajj on behalf of my maternal grandmother but my father objected and said that her children had more right to do that. Do I have to obey him in this matter?.
Praise be to Allah.
    Obedience to parents is obligatory in matters where it will 
    benefit them and not cause you any harm. But if they tell you not to do 
    naafil acts of worship, we must examine the matter further. If they need you 
    to do something and you will not be able to do it because you are 
    preoccupied with this naafil act of worship, then you should obey them, such 
    as if your father says to you, “O So and so, wait for the guests and do not 
    pray a naafil prayer.” In this case you have to obey him because there is a 
    justifiable reason for it. But if he tells you not to pray Duha because he 
    does not like such things, and he does not like naafil acts of worship 
    because he does not have strong faith, then you should not obey him. But you 
    should try to be tactful with him as much as you can, in the sense of hiding 
    the good deeds that you are doing. 

    So we say to this questioner: Do Hajj on behalf of your 
    grandmother, and if your father tells you not to, say, “It doesn’t matter” 
    and do Hajj anyway. There is no lying involved in this if you are able to 
    dissimulate. Dissimulation means saying to him, “I will not do Hajj”, 
    meaning next year. This father is telling you to sever family ties, or he is 
    ignorant. So you can tell him, “I will not do Hajj on her behalf, just to 
    please you,” meaning that you will not do Hajj on her behalf next year, 
    because you are going to do it this year.  

    Similarly some mothers, if they see that their son has a good 
    relationship with his wife, will say, “O my son, (choose) either me or her,” 
    so that he will divorce her. And some fathers may not get along well with 
    their son’s wife so they tell him to divorce her, but he should not divorce 
    her. A man asked Imam Ahmad (may Allaah have mercy on him): “My father told 
    me to divorce my wife but I love her.” He said: “Do not divorce her.” The 
    man said: “When Ibn ‘Umar's father told him to divorce his wife, he asked 
    the Prophet  (peace and blessings of Allaah be upon him) about that and 
    he said, ‘Divorce your wife.’ So he told ‘Abd-Allaah ibn ‘Umar to obey his 
    father and divorce his wife.’” Imam Ahmad said to him sternly: “Is you 
    father ‘Umar?!” This means something, because ‘Umar (may Allaah be pleased 
    with him) only told his son to divorce his wife because he saw a legitimate 
    shar’i reason for doing so. But perhaps your father has a personal reason 
    because there is some problem between him and your wife. End quote.