Can a father disinherit his son after he has signed two documents: one for waiver and one for receipt of a sum of money?

No one is permitted to prevent an heir from their right to inheritance, for Allah Almighty says, after detailing the shares of inheritance: "These are the limits [set by] Allah, and whoever obeys Allah and His Messenger will be admitted by Him to gardens beneath which rivers flow, abiding eternally therein; and that is the great attainment. And whoever disobeys Allah and His Messenger and transgresses His limits - He will admit him to a fire, abiding eternally therein, and he will have a humiliating punishment." The intent is that these details concerning inheritance are the limits of Allah, which must be adhered to.