When We Were Dragons by Brandon Berntson

Justin Silas is a dragon who can change into mortal form at will. When his world, Paramis, collides with Earth with the aid of powerful magic, destruction ensues.

Cerras, a giant made of amber quartz and god of Paramis, stirs, restless in his slumber.  After breathing life into Paramis, he willed himself to sleep for thousands of years, allowing dragons and mortals to reign in his stead.

But all is not well.

Lane, a traitorous dragon, is turning dragons against mankind and mankind against dragons. She longs to usurp the throne of Cerras, hoping to destroy the giant god with his own power.

Yet fate lies with Charlie Underhill, a boy from Earth, who - since the collision - harbors a special kind of magic all his own.