Genesis 10:21

And to Shem also was born a son. He is the father of all the sons of the Hebrews, the brother of Japheth, great in the fear of the Lord.

AND UNTO SHEM, TO HIM ALSO WERE CHILDREN BORN. Since Scripture delayed the narration of the generations of Shem and related the generations of his younger brother as if Shem had no children, it therefore says here, to him also.

THE FATHER OF ALL THE CHILDREN OF ‘EBER.’ This means that he was the father of all who dwelled beyond (eber) the Euphrates River, which was the place of Abraham’s family. But it is not possible that Eber in this context be the name of the person who was the father of Peleg for why would Scripture connect the children of Eber with Shem [moreso than with any of his other offspring]?

THE BROTHER OF JAPHETH THE ELDER. It is the way of Scripture to record a younger brother beside the oldest of his brothers, and not beside a younger one than himself., rather than “the elder brother of Ham.” And similarly, we find the verse, Miriam the prophetess, the sister of Aaron. The reason for mentioning this altogether is to state that he [Shem] is the brother of the honorable Japheth, comparable to him in distinction. Scripture thus declares that the reason it delayed telling of his genealogy is not because Ham was more distinguished than he.
It appears to me that “the elder” is descriptive of Shem, is descriptive either of Japheth or of Shem. If so, the elder refers to age in years for in my opinion Shem was the oldest of the brothers while, in the opinion of most commentators, Japheth was the oldest. It is also possible to explain the elder as referring to distinction, and it may also be descriptive of either of them.” meaning that he was the older brother of Japheth, as Ham was the youngest of all, even though Scripture mentioned Japheth first. So, likewise, in all places in Scripture, the descriptive noun refers to the subject spoken of, such as: Isaiah the son of Amoz the prophet, [the term “the prophet” refers to Isaiah, who is the subject of the narration]; Unto Hobab, the son of Reuel the Midianite, Moses’ father-in-law, [here, “Moses’ father-in-law” refers back to Hobab]. But Rashi wrote: “The brother of Japheth. It does not state “the brother of Ham” because these two [Shem and Japheth] honored their father whereas Ham put him to shame.” This interpretation also declares that the sense of the verse is that Shem is the brother of the righteous brother, and not the brother of the wicked brother even though he is counted after him.

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

גם 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.”

ולשם, the reason that the descendants of Shem are listed last in this chapter is that the story will continue with the lives of the patriarchs, all of whom are descended from Shem. The Torah is trying to preserve continuity. As to the unusual phrase

גם הוא, the Torah adds these words as if to say that we should not think that seeing that up to now we heard about Shem always first, that the fact that we had not heard about him in this chapter means that he had not been blessed with offspring. Not only did he have sons, but he became the founding father of all the tribes descended from עבר, who in turn became the ancestor of the patriarchs. In fact, his descendants were even more important than those of his older brother Yaphet, described as הגדול, the great one. The attribute הגדול, when used in conjunction with Yaphet may be understood as Shem sharing his brother Yaphet’s distinction of doing good deeds. The Torah invites you to note that Shem has not been compared to Cham, neither in seniority nor in quality of lifestyle. The word הגדול may equally well be understood as “the great one,” i.e. in deeds.

יולד, a somewhat strange word when we would have expected יולדו in the plural mode, seeing that a number of sons were born to him. The singular is used to cover the subject of progeny being born. It has been use in a similar manner in verse 25, when speaking of Ever’s progeny.