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

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.