Is a father's will for his son to marry his paternal cousin and not to marry another girl legally binding, especially with the son's desire to marry the girl he loves and his paternal cousin's rejection of him?

Obeying parents in marriage is a form of righteousness and benevolence, as long as the girl's soul does not resist. If it does, you are not obligated to obey them, nor will you be considered disobedient if you refuse. As Shaykh al-Islam Ibn Taymiyyah said: "Neither of the parents has the right to compel their child to marry someone he does not desire, and if he refuses, he is not considered disobedient." You should gently persuade your parents to accept another girl who possesses religion and good character, while being careful to avoid any unlawful relationship with her.