In Philippians 2:22, Paul says Timothy served with him in the gospel like what?

Answer: A child with his father

Paul hoped to send Timothy, whose proven worth they knew: as a child with his father (KJV: a son with the father) he served with Paul.

“But you know Timothy’s proven worth, that as a child with his father he has served with me to advance the gospel.”— Philippians 2:22 (BSB)