פירוש על בראשית 17:25

בהמולו את בשר ערלתו WHEN HE HAD THE FLESH OF HIS FORESKIN CIRCUMCISED — With Avraham, [the word], et is not stated [as it is here with Yishmael], because the only thing he was missing was the cutting of the flesh, as [his foreskin] was already flattened through sexual relations. But Yishmael - who was only a child - was required to cut his foreskin and open [and push back] the circumcision. Hence about him, et is stated. (Genesis Rabbah 47:8)

וישמעאל בנו בן שלש עשרה שנה, seeing the Torah had mentioned Avraham's age at the time of his circumcision, Ishmael's age was also mentioned as a mark of honour for him. After all, we know how old Ishmael was at the time, the Torah having told us that Avraham his father was 86 years old when he was born (Genesis 15,16). Perhaps the Torah underlines the fact that Avraham circumcised his son Ishmael, as if he had not been circumcised by that age, he himself is obligated to either circumcise himself or to arrange for him circumcision. Ishmael was just young enough for this obligation still to be that of this father.

Therefore, concerning him את is said. The words את and גם always come to add something. Although it is stated in Yevamos 71b that Avraham was not commanded to do פריעה, he did it on his own just as he kept the whole Torah on his own. But regarding מילה he waited since he knew he was destined to be commanded, and then he could do it in fulfillment of a command, [which is more praiseworthy]. (Kitzur Mizrachi)