坊っちゃん
ぼっちゃん · bocchan · JLPT N2 · noun
son
坊 (boy) + ちゃん (cute suffix) = young master, used for someone else's son
坊 originally meant a Buddhist monk's residence, later a boy. Adding ちゃん makes it affectionate, so it is used for another family's son, as in Natsume Soseki's novel Botchan.