The Jews had falsely claimed to be sons of God based upon their physical lineage through Abraham, and were presuming upon the rights of sons, which they, as slaves, did not possess. There is a position a son has in a household which can never be altered. He is a permanent dweller and may never be ejected. As Barclay points out, “No man can unmake a son; a son is always a son.” But not so with a slave; he may be ejected from the household at any time. Compare Ishmael and Isaac (Gen 21:10). A son has rights; a slave has none.

Gen 21:10  and she said to Abraham, “Get rid of that slave woman and her son, for that slave woman’s son will never share in the inheritance with my son Isaac.”

But Jesus is not just a son; He is the Son. And He “abideth ever” which indicates His eternal existence.