Komentarz do Rodzaju 21:3

בנו הנולד לו, His son who had been born to him. Inasmuch as Abraham had another son, Ishmael, the Torah records here that Abraham stressed that this was the son who was fit to bear his name and who would fulfil the aspirations he had for him. No such comment is recorded at the time Ishmael was born. The Torah's addition אשר ילדה לו שרה, whom Sarah bore for him, is an allusion to the fact that both had constantly prayed. Abraham had asked that all his descendants should be issue of his righteous wife Sarah, whereas Sarah had also prayed to be the mother of all of Abraham's children. The word שרה refers to her prayer, whereas the words אשר ילדה לו, refer to Abraham's prayer.

ויקרא, he complied with G’d’s instructions issued in 17,19 to call the son that would be born to him by Sarah Yitzchok.

Who was born to him: The lengthier expression than what is written (Genesis 16:15) regarding Yishmael - "the name of his son whom Hagar bore" - is coming to teach through this that at the time of the naming, Avraham publicized that [Yitzchak] was his son, to remove [any doubt] from people's minds.

And he called: Behold, God commanded Avraham, "your wife Sarah will bear you a son, and you will name him Yitzchak" (Genesis 17:19) - and he had to implement this immediately on the day that he was born; and He commanded him to circumcise him on the eighth day. So Avraham implemented, "And he called his son's name," immediately; and he circumcised, "when he was eight days old" (Genesis 21:4). And about both of them, it stated, "as the Lord had commanded him." And it stated, "that was born to him, to which Sarah had borne to him" - as it was written, "your wife Sarah will bear you a son" - that is to say, a son who will be the heart and treasure that will be descended from you. As this could only be from the righteous woman, Sarah.

הנולד לו אשר ילדה לו שרה, in beiden Beziehungen: dass ihm, dem Hundertjährigen, noch und erst ein Sohn geboren wurde, und dass Sara, die neunzigjährige, noch und erst Mutter ward, war die Geburt dieses Kindes, zumal nach allem, was sich an dieselbe knüpfte, צחק, etwas den natürlichen Verhältnissen zufolge Lächerliches (siehe oben zu Kap.17. 17), und als Abraham seinen Sohn יצחק nannte, war ihm beides lebhaft gegenwärtig.

Another reason for this nuance is that the name יצחק, i.e. joy, reflects the fact that an old woman had given a son to an old man. Sarah spelled this out in greater detail in verse 6.