The Replacement, by Brenna Yovanoff

(@brennayovanoff) Though he lives in the small town of Gentry, Mackie comes from a world of tunnels and black murky water, a world of living dead girls ruled by a little tattooed princess. H

He is a Replacement -- left in the crib of a human baby sixteen years ago. Now, because of fatal allergies to iron, blood, and consecrated ground, Mackie is slowly dying in the human world...
(From the YouTube description)