Jesus was not always the Son of God, he became the Son of God when he was conceived by the Holy Spirit in the womb of Mary. In heaven he was called the Word of God (John 1:1), He never co-existed along side God, he was in God as the Word of God. There is one God, not two and this God manifest himself through three personalities, first through his Image, and then through his Word, and lastly through his Spirit. The Father, the Son & the Holy Spirit. The Word of God is a person, and this person came into time in the form Jesus, “The Word became flesh and made his dwelling among us John 1:14.”
When God speaks, whatever he said becomes the seed to produce the life of what he said, if he says to an empty earth, “let there be trees”, his Word becomes the seed required in the ground to produce trees. The Word of God spoken to Mary was the seed which produced the baby Jesus through the Holy Ghost. The reason Jesus is called the son of God is because he was not born of human seed or sperm, his seed was the Word of God, and the Word of God is God, so God gave birth to him. Biologically they tell us that, the human body comes from a woman, but the life of every child comes from a male sperm. The Body of Jesus came from Mary, but his life came straight from from God Almighty. The Father had never had a son before Jesus, he gave birth to his very own Son through the Word. Hebrews 5:5 God said to him, "You are my Son; today I have become your Father."