What is the ruling of Islamic law concerning an adopted child of unknown parentage remaining with his adoptive family, given that adoption is forbidden, and knowing that he has been officially registered under the name of the family that adopted him?

The care provided by a couple for a child is a noble act, but adoption (tabanni) is forbidden and requires repentance. Adoption does not entail the rulings of filiation, such as inheritance and mahramiyya (prohibition of marriage), and the child's lineage must be changed. He can be called a brother in faith or by a general kinship term if there is a fear of embarrassment. It is not obligatory to expel him from the house, but the paternal uncle's wife is a stranger to him. If something is written down for him as a sale and purchase, mere writing does not confer ownership. A gift is not owned until it is taken possession of, and if it is a bequest (wasiyya), it is only effective within one-third of the estate, and it is not permissible to intend to deprive heirs.