"In addition to the tender love that is contained in this expression there lies in it the thought of immaturity: Onesimus is only a child as yet and in this condition needs much tender care lest his young spiritual life suffer or die." [1][source]
"This is the one-time self-righteous Pharisee, the heir of Jewish exclusiveness, and he is speaking of a Gentile, and a Gentile slave at that, from the very dregs of Roman society—yet he can refer to him as a son. So his statement (Col. iii11) that "there is neither Greek nor Jew ... bond or free" [2] is no empty formula but reflects the attitude of heart to which he himself had been brought by God." [1][source]
Paul had led Onesimus to Christ while Paul was in confinement. The apostle softened the unpleasantness that the mention of Onesimus" name would have produced by making a pun. "Useful" had been "useless" to Philemon , but now he was living up to his name. He had proved useful to Paul and he could be useful to Philemon. There was no need for Paul to identify exactly what sin Onesimus had committed against Philemon. Instead of magnifying it he minimized it (cf. 1 Peter 4:8). [source][source][source]
"Achrestos [4] designates Onesimus with reference to his flight and the time before his conversion. Apparently he was useless even before he ran away. He was a Phrygian slave and as such "had confirmed the popular estimate of his class and nation by his own conduct" [5] since Phrygian slaves were proverbial for being unreliable and unfaithful." [6][source]
"(The name Philemon means "affectionate" or "one who is kind." If the slave was expected to live up to his name, then what about the master?)" [7][source]