The Replacement by Brenna Yovanoff

Mackie Doyle is your typical sixteen year old boy, he plays bass guitar and gets crushes on girls like other boys, but Mackie isn't really a normal boy; he is slowly dying. He is a Replacement, a creature from another world entirely left in place of a real human baby sixteen years before. Being allergic to blood, iron and consecrated ground he is having trouble surviving. Mackie has two options: go to his own true world and survive, or stay in this one and surely die.