Is it permissible to give Zakat al-Mal (obligatory charity) to a son who cannot find anyone to support him because his father refuses to spend on him?

The maintenance of a son who is not self-sufficient is obligatory upon his wealthy father. The mother's spending on him from her own money will not lose her reward if she seeks the face of Allah by it. The mother must advise the father and remind him of his duty to spend, and that neglecting those under one's care is a great sin. It is not permissible to give the son from the mother's Zakat, because the father is obligated to provide for him.