1)
Why does the Torah find it necessary to add that Yishmael was the son of Hagar ha'Mitzris Shifchas Sarah?
1.
Rashbam and Ramban (Bereishis 25:19): To stress that Yishmael was not the real son of Avraham, in contrast to Pasuk 10, where it adds "Avraham bore Yitzchak" - since he was his real son. 1
2.
Ha'Emek Davar: Regarding Avraham, Yishmael is called a son. Regarding Sarah and Yitzchak, he is called a Ben Shifchah.
1
Hadar Zekenim (on 25:19): The son of a Nochris is not called your son, rather, her son (Kidushin 68b).
2)
למה מיחס כאן הכתוב את ישמעאל לאברהם?
1.
גור אריה (לקמ' פסוק יט): תפארת בנים אבותם, ולכן בא הפסוק ליחס את בני ישמעאל לאברהם.
