Psalm 2 and 2 Samuel 7 provide royal sonship background, while divine overshadowing recalls manifestations of God's presence.
Jesus' sonship is not presented merely as an honorary title. Luke relates it to his unique origin, holiness, everlasting reign, and relationship to the Most High.
Royal sonship alone would not prove deity, but Luke fills the title with features that make Jesus' sonship unique and foundational to his identity.