Is a Muslim son who was adopted by a non-Muslim family obligated to care for them and treat them kindly, and what about his biological parents if he later learns of their existence?

In Arabic, "adoption" (تبني) has two meanings: The first is raising a child and caring for them without changing their lineage. This is permissible and commendable. The second meaning is raising a child and attributing their lineage to the adoptive family. This is forbidden in Islamic law due to the Almighty's saying: "Call them by [the names of] their fathers; it is more just in the sight of Allah." Lineage entails many legal rulings. Despite the prohibition of changing lineage, showing kindness to those who raise and care for a child is obligatory, and reciprocating good with good is part of Islam's character and guidance. As for searching for the biological parents, it is an innate matter and a religious obligation due to the rulings and psychological effects that result from it.