Wargod's Apprentice

At fifteen, Lex, who has been homeschooled his entire life, begins attending high school. Although he is gifted and ahead of his peers academically, he's awkward and uncomfortable socially-that is, until he discovers football and its violence, and it seems he has a gift for both.
When Lex's family dies in a fire, Lex goes to live with his fabulously wealthy grandfather who apprentices him to an eccentric named Dr. Bernard Polemarchos.Bernie is a Wargod, one of a race of aliens who travel in time, refighting the great battles of history to protect earth from the evil Others-who, Lex discovers, are responsible for his family's death. As Bernie teaches Lex how to be a warrior, soon the time will come for the young man to avenge the murders of his loved ones and take his place in history.
In this novel, a boy whose family is murdered goes to live with his grandfather and learn the art of war from a member of an alien race who fights to protect the earth.