Hadith Commentary:Benefits and Issues: From this hadith, it is understood that it is permissible for a person to address a younger boy with affection, love, and kindness by saying, "O my son" (ya bunayya, ya buni), or "O my child" (ya waladi), just as it is permissible to address a peer on this basis as "O my brother" (ya akhi), and it is correct to address someone older than oneself as "O my uncle" (ya ammi).
Source: Tuhfat al-Muslim: Commentary on Sahih Muslim, Page: 5623
Narrated Anas bin Malik: The Prophet ﷺ said to him: My sonny. Abu Dawud said: I heard Yahya bin Main praising the transmitter Muhammad bin Mahbub, and he said: He transmitted a large number of traditions.
Shaykh Umar Farooq Saeedi
Benefits and Issues: There is no harm in affectionately calling someone else's child "son" or "my son." The command in Surah Al-Ahzab that they should be called by their fathers' names is regarding adopted children, so that their real lineage is not erased. Otherwise, saying this affectionately or figuratively in this manner is permissible.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 4964