What is the ruling on money taken by a son from his father to buy a car, and is it considered a debt that must be repaid, or a gift or assistance?

As long as the father does not want his son to return the money, the son is not obligated to return it. The debt is waived for him, his responsibility is cleared, and there is no sin upon him for accepting his father's assistance. The father is not required to give his other children the same amount he gave this son if he gave it to him due to a need specific to him. The son, however, must honor his father and be good to him.