Commentary for Genesis 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.