2 And again on hearing of a “Son,” think not of an adopted son but a Son by nature, an Only-begotten Son, having no brother. For this is the reason why He is called “Only-begotten,” because in the dignity of the Godhead, and His generation from the Father, He has no brother. But we call Him the Son of God, not of ourselves, but because the Father Himself named Christ His Son: and a true name is that which is set by fathers upon their children.
Source: Catechetical Lectures (New Advent)