What should I do, as I borrowed a sum of money from my son and swore by Allah that I would repay it, but he refused to take it; so, should I fast for three days, and should I accept the money if he gives it to me, and should I give him the money and then he returns it to me out of devotion to the oath?

A Muslim should not swear excessively. If someone makes an oath to another, it is recommended for the one to whom the oath is made to fulfill his brother's oath. Therefore, your son should accept your religion from you, and then he may gift you whatever he wishes. By doing so, he would have fulfilled your oath and saved you from breaking it. If he insists on not accepting, you are obligated to pay the expiation (kaffarah), which is feeding ten poor people, or clothing them, or freeing a slave. If one cannot find these, then it is fasting for three days.