Is it permissible for a father to dispose of his son's money, even if it is at the expense of his grandchildren's future, relying on the saying "You and your money belong to your father," and would the son's opposition in this case be considered an act of disobedience, especially if he proposes an alternative?

It is permissible for a father to take from his son's wealth what he needs, provided two conditions are met: that it does not harm the son, and that this money is not given to another child. Based on this, it is not permissible for the father to compel you to share your wealth with your brother, as this falls under the ruling of taking the money of one son to give it to another, which is forbidden; because the father is prohibited from favoring one of his children over another. Your disobedience to your father's command in this case is not considered disobedience (uquq), but you must be gentle with them and try to persuade them, seeking the help of religious and righteous people to clarify the Sharia ruling for them. You can help your brother by providing a salary or other financial assistance that you are comfortable with.