Albert Barnes Commentary Philemon 1:13

"whom I would fain have kept with me, that in thy behalf he might minister unto me in the bonds of the gospel:" — Philemon 1:13 (ASV)

Whom I would have retained with me, that in your stead. "That he might render me the service which I know you would if you were here." The Greek is, "for you;" (uper sou); that is, what he should do for Paul might be regarded as done by Philemon himself.

He might have ministered to me. He might have rendered me assistance, (diakonh); namely, in such a way as one who was in bonds would need.