In the Old Testament, "son of God" is a title given to the angels, the Chosen People, the children of Israel, and their kings. It signifies an adoptive sonship that establishes a relationship of particular intimacy between God and his creature. When the promised Messiah-King is called "son of God", it does not necessarily imply that he was more than human, according to the literal meaning of these texts. Those who called Jesus "son of God", as the Messiah of Israel, perhaps meant nothing more than this.
Source: Catechism of the Catholic Church (Vatican.va)