Is repentance permissible for one who committed adultery with his sister and had a child with her, and what should he do with the child resulting from this relationship?

The man committed a grave sin, but he has a chance for repentance, for Allah Almighty says: "Indeed, Allah does not forgive association with Him, but He forgives what is less than that for whom He wills," and Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" As for the child, he is a child born out of wedlock and is not attributed to him; rather, he is attributed to his mother, and she is responsible for his custody, upbringing, and financial support.