Is it permissible for a son to withhold his father's money and prevent him from accessing it, under the pretext of his father's previous misconduct, while the father is in need of the money for his medical treatment and to provide for his minor children?

It is not permissible for a son to prevent his father from disposing of his wealth, as that would be an act of disobedience (uquq). If the father does not manage his wealth well, the son should advise him in a kind manner and seek the help of someone who can influence his father. If the father reaches the point of squandering money, the son may refer the matter to the Sharia judge for consideration.