Is it permissible to give Zakat al-Mal (obligatory charity on wealth) to one's son to help him with marriage expenses?

A father must help his son who is unable to marry to achieve chastity, because providing for him is a mandatory expenditure. A father is not permitted to give his zakat to his son if that would lead to the forfeiture of his mandatory expenditure towards him. However, if the father is unable to provide for him, then it is permissible for him to give his zakat to him. This is the view of Sheikh Al-Islam Ibn Taymiyyah. If the son is strong and capable of earning enough to marry, then zakat should not be given to him under any circumstances.