What is the religious duty towards a father who abandoned his son forty years ago, never acknowledged him, and treats him harshly, knowing that his marriage to the mother was عرفيًا (customary/unregistered), and that the son carries the name of his grandfather (the father's father) in official documents?

If a father disowns his children with the intent of denying their lineage, it is forbidden. For, in an urfi marriage that fulfills its conditions, the child is attributed to the father, even if the marriage itself is invalid in reality. If the disowning is intended to sever ties with them, it is one of the grave sins. The father is obligated to raise his children, care for them, and provide for them. And the children are obligated to honor their father and be kind to him as much as possible. If he expels them, they are not obliged to visit him in his home; it suffices to contact him and pray for him. There is no harm in attributing oneself to the grandfather.