Fire Touched (Mercy Thompson #9) by Patricia Briggs
Synopsis from GoodReads:
Tensions between the fae and humans are coming to a head. And when coyote shapeshifter Mercy and her Alpha werewolf mate, Adam, are called upon to stop a rampaging troll, they find themselves with something that could be used to make the fae back down and forestall out-and-out war: a human child stolen long ago by the fae.
Defying the most powerful werewolf in the country, the humans, and the fae, Mercy, Adam, and their pack choose to protect the boy no matter what the cost. But who will protect them from a boy who is fire touched?