Dark Rise Series by C.S. Pacat

From Book One:

The ancient world of magic is no more. Its heroes are dead, its halls are ruins, and its great battles between Light and Dark are forgotten. Only the Stewards remember, and they keep their centuries-long vigil, sworn to protect humanity if the Dark King ever returns.

Sixteen-year-old dock boy Will is on the run, pursued by the men who killed his mother. When an old servant tells him of his destiny to fight beside the Stewards, Will is ushered into a world of magic, where he must train to play a vital role in the oncoming battle against the Dark.