Sforno on Genesis 10:21/ספורנו על בראשית י׳:כ״א

1ולשם יולד, the formulation suggests that the son born to Shem resembled his father in his good qualities, the son in question being עבר.

2גם הוא אבי כל בני עבר, although, as we know from the description of Avraham as an עברי, a Hebrew, (a descendant of Ever) i.e. as someone believing in G’d the Creator being the sole G’d, and it might appear that Ever initiated this adherence to monotheism in his time, Shem, the father too, was a founder of preaching monotheism in his time. Anyone teaching and instructing others, especially in a way of life and philosophy, is called אבי, “father of.” We have come across this term as possessing such meaning already in Genesis 4,21 when Yuval was introduced as the inventor and teacher of how to play musical instruments featuring strings. Just as the teachers, mentors are called “fathers,” so the disciples are called “sons,” בנים, as in Samuel I 10,12 בני הנביאים, “the disciples of the prophets.”