Is it permissible for a poor widowed mother to take money for her sustenance from her disbelieving son?

This money is lawful (halal) because it is permissible to accept a gift from a non-believer (kafir). Imam Al-Bukhari dedicated a chapter titled "Accepting Gifts from Polytheists" and included the hadith of Anas therein: "A Jewish woman brought a poisoned sheep to the Prophet, peace and blessings be upon him, and he ate from it." It is obligatory upon you to advise your brother, invite him to the religion, encourage him towards faith, and help him with supplication, especially the supplication of a mother.