What is the ruling on a father compelling his son to marry his paternal cousin on the pretext that it was his grandmother's dying wish, and threatening him with his displeasure if he does not marry her, especially since the son does not desire her? Is the father's displeasure permissible in this case? And what is the legitimate way to convince the father?

The grandmother's will to marry her grandson to his female cousin is not binding. A father does not have the right to force his son to marry someone he does not desire, and the son's disobedience in this case is not considered disobedience to parents (Uquq). This should be explained to the father. If the father insists on marrying his son to his female cousin and no harm will result to the son, then it is preferable for him to marry her. Otherwise, there is no sin upon him if he marries someone else. The son must honor his father and treat him well, because the father's right is great, and honoring him is among the best acts of drawing closer to Allah.