Is the one who withholds a financial gift from his father considered undutiful to his parents, especially if the father is not in need of the money, and the son's withholding of the gift is due to his own need for the money to cover his family's expenses and debts and to provide them with housing?

There is no sin upon you for withholding a gift from your father, and this is not considered undutifulness as long as both your parents are self-sufficient, for their sustenance is not obligatory upon you. If they are in need, and you do not have more than what suffices your needs and the needs of your dependents, then their sustenance is not obligatory upon you; due to the statement of Ibn Qudamah: "Three conditions are required for the obligation of maintenance: First: That they be poor, having no wealth or earnings that suffice them from the maintenance of others... Second: That the one upon whom the maintenance is obligatory has what he can spend on them, in excess of his own maintenance." This is based on what Jabir narrated, that the Messenger of Allah, peace and blessings be upon him, said: "If one of you is poor, let him start with himself; if there is an surplus, then for his dependents; if there is still an surplus, then for his relatives." However, you must be dutiful and kind to your parents, and be gentle in apologizing to him.