The Fallen, tome 1 & 2 : The Fallen & Leviathan
Thomas E. Sniegoski2003

On his eighteenth birthday, Aaron begins to hear strange voices and is convinced he is going insane. But having moved from foster home to foster home, Aaron doesn't know whom he can trust. He wants to confide in the cute girl from class, but he's afraid she'll confirm he's crazy. Then a mysterious man begins following Aaron. He knows about Aaron's troubled past and his new powers. And he has a message for Aaron: As the son of a mortal and an angel, Aaron has been chosen to redeem the Fallen. Aaron tries to dismiss the news and resists his supernatural abilities. But he must accept his newfound heritage - and quickly. For the dark powers are gaining strength, and are hell-bent on destroying him...