1. Who changed the IOUs? If it is changed with the son's consent, the loan will be transferred to the son's name and the son can pay off the debt directly.

If it is changed without the son's consent, the son still disagrees with the debt of the deceased, not the son's debt.

Even if his son doesn't agree to change his name: if his son inherits the inheritance of the deceased, he can ask his son to repay, but only within the scope of his son's inheritance.