Abinadi had an intimate knowledge of the close relationship between the Father and his Beloved Son. In fact the Son would personify that relationship and represent them both when he came to the earth in his role as a Redeemer.
What Abinadi is teaching is this: because Jesus would be conceived by the power of the Father he would be of the Father, and because he would come in the flesh as the offspring of the great Elohim, he would be called the Son.