1)
Why does it say "Af Ani"?
1.
Radak: Just like he will call me Father, I will call him a son, and even [My] Bechor. Malbim - also, because he calls Me "Keli v'Tzur Yeshu'asi", I will make him supreme over all kings.
2)
How will He make him a Bechor?
1.
Rashi: I will make him great.
2.
Radak: Just like a Bechor has attributes over other sons, so he will be supreme over all kings - "va'Shem Nasan Pachdo Al Kol ha'Goyim" (Divrei ha'Yamim I, 14:17).
3.
Malbim: I give to him all natural powers and all his needs, like a father does for his Bechor.
