Hebrews 1:5-6 states that the WORD (John 1:1 "...and the WORD was GOD") became the SON when he entered the world, "Thou art my son this day have I begotten thee...I will be to him a father and he shall be to me a son." This passage clearly reveals a time when the WORD (Yahweh) became the SON.
Isaiah wrote that the WORD was the everlasting FATHER (Yahveh/Godhead). Micah 5:2 revealed that Messiah would be YAHWEH, who was from "everlasting."
The doctrine of the "eternal sonship" of Christ has no basis in scripture but rather has it's genesis in Aryanism which teaches that Jesus, the one who created all things, (Colossians 1:16-17) is a subordinate created being. Yahweh created all things, by himself--alone, Isaiah 44:24. Jesus is Yahweh.
The Messiah (Immanuel/Yahweh with us) was born to a virgin. God became a man, and that man was the Son of God. God became the Son when he was born into this world as a human being, fully God and fully man (The fullness of the Godhead dwelt in our savior in physical form (Colossians 2:9).