noun
adopted son
A son who has been legally adopted into a family. The term is straightforward and neutral.
彼はもらい息子だが、実の子のように育てられた。
He is an adopted son, but he was raised like a biological child.
養子 is the more formal and general term for an adopted child, while もらい息子 is a colloquial, gender-specific term for an adopted son.
Compound of もらい (receiving) from 貰う (to receive) and 息子 (son). The term literally means 'received son'.