Verse 1’s declaration that “God himself shall come down among the children of men” is the important concluding statement after Abinadi’s quotation of Isaiah 53’s suffering servant text. That text spoke of a mortal who would take upon himself our iniquities. Then Abinadi declares that God himself, or Jehovah himself, would be that man.
The rest of the verses are attempting to clarify the Nephite understanding of God. First, as part of the house of Israel, Jehovah is their God. When that God changed habitation, when that God condescended to come to earth, that shift gave him a different designation.
Abinadi clarifies that there is no difference in person between Jehovah in the heavens and Jehovah as a mortal. It is simply the change of location and subjection to humanity that creates the designation as the Son of God. He is both the Father and the Son simultaneously. The Father as God, the Son as mortal. Nevertheless, it is the same being. It is the state that changes the designation.
It is also possible that the terminology created a different type of designation for the mortal Messiah. While modern readers easily see Son of God as a title for Jesus Christ, it is possible that the use in the Book of Mormon derives more from the tradition surrounding Israelite kingship.
In Psalms 2:7, the Lord says to the king: “Thou art my Son; this day have I begotten thee.” The Israelite kings joined with their ancient Near Eastern counterparts in considering themselves sons of god. If this is the meaning, then the import of this change in state is not to create a new biological relationship, but rather a new regnal designation for the coming atoning Messiah.