We have seen, and bear witness, and show unto you that eternal life, which was with the Father, and was manifested unto us.
Jesus always was and always will be the Son of God. He didn’t become the Son when He was born here in Bethlehem. He became a human being, yes. But He was always God’s Son. John knew Jesus as the eternal Son, and he wrote this part of Scripture as a first hand witness of being with, and seeing, the eternal Son of God - as a Man.