Is it permissible, in Islamic law, to secretly take money from one's father to cover the expenses of hair loss treatment, especially given the father's refusal to spend on his son's treatment?

It is impermissible to take money from the father without his knowledge for hair treatment because it is not a necessity, and because the basic principle is the inviolability of a Muslim's wealth unless it is given willingly, and this includes the father's wealth. Also, the majority of scholars hold that the expenses of an adult son capable of earning a living are not obligatory upon the father.