The Last Words of Will Wolfkin by Steven Knight

It's funny. If you're born a certain way, you don't really understand how it is to be any other way.

So it has been for Toby Walsgrove—paralyzed since birth, unable to move or talk, with no known family, he has spent his entire life at a Carmelite convent in London. That is, until the day that his cat, Shipley, starts talking to him. Shipley has been watching over Toby his whole life and tells him they must go to Langjoskull, a city of exiles buried deep below the surface of Iceland. Because Toby is no ordinary boy—he's a descendant of the great king Will Wolfkin, and his kingdom needs him.

Toby has never wielded a sword that can stop time. He has never shifted into his kin creature. He has never even walked on his own two legs before. Ready or not, though, he has a destiny, a responsibility, even a family—and not all of them are happy to meet him. . . .