It Is Permissible To Say: "O My Son" To Someone Other Than One's Son, And It Is Recommended To Speak Kindly
حَدَّثَنَا مُحَمَّدُ بْنُ عُبَيْدٍ الْغُبَرِيُّ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ أَبِي عُثْمَانَ، عَنْ أَنَسِ بْنِ، مَالِكٍ قَالَ قَالَ لِي رَسُولُ اللَّهِ صلى الله عليه وسلم " يَا بُنَىَّ " .
Anas b Malik reported that Allah's Messenger (ﷺ) addressed me: O My Son.