Does a child born of a common-law marriage inherit from his deceased father, is he considered illegitimate, and is the questioner sinful for not taking care of him?

For a marriage to be valid, the presence of a guardian (wali) and witnesses is stipulated. If one of these conditions is absent, the marriage is invalid. However, the child is attributed to the father due to the presumption of a marital union (shubhah). This child is attributed to this man in this case and has a share in his inheritance, should he leave any wealth, according to the words of Allah Almighty: "Allah instructs you concerning your children: for the male, what is equal to the share of two females." And if the father neglects the child's rights, he is accountable before Allah, as the Prophet, peace and blessings be upon him, said: "It is sufficient sin for a man to neglect those whom he sustains."