We have to take Jesus as having been a Son eternally, because The Father is a father eternally. The Father's fatherhood did not begin only when the Word became flesh: His fatherhood is eternal, because he has a Son who is eternally with Him. If Jesus is the Son only according to his humanity, then the Father was not always Father, having had no only-begotten Son until the Word became flesh. This is unacceptable, because Jesus' relationship with His Father is from all eternity, without beginning.
In other words, the relationship between the Father and the Son does not change. It is eternal. Christ was with the Father (His words), which means that He was a Son of His Father. This relationship between them exists eternally, without beginning. There are not two persons in Christ - one Divine and one human - there is only one, the Divine Son of God.
"And now, O Father, glorify thou me with thine own self with the glory which I had with thee before the world was." (John 17:5)
"The Word became flesh and made His dwelling among us. We have seen His glory, the glory of the one and only Son from the Father, full of grace and truth." (John 1:14)
His glory as Son of the Father is from "before the world was".