Once Upon a Changeling by V. J. Chambers

Things started going wrong for Russ Knight last year.
He found out his girlfriend was cheating on him when she gave birth at the junior prom and left the baby to die in a trashcan. Russ didn’t even know she was pregnant. Hell, he thought they were saving themselves for each other.
Now, locked in her padded cell, his girlfriend is screaming that she was just trying to get the faeries to give her baby back. Russ doesn’t buy it. But to shut her up, Russ tries a trick the old legends say will work, and the baby starts swearing and begging to be taken back to Faerie. He’s got to accept the truth: the kid he’s raising is a faerie changeling.

The faeries are going to sacrifice the real baby at the Equinox if Russ can’t get them to switch again. But the only person who can get the baby back is a biological parent, and Russ’ girlfriend certainly can’t help. To save an innocent life, Russ will face ancient faeries with razor-like teeth, wrestle snarling skeleton dogs, and, maybe worst of all, track down every guy his girlfriend was sleeping with last year in the hopes of finding the baby’s real father.

And apparently, the father could be half his senior class.