Lord Loss (The Demonata #1)

Grubbs Grady has stiff red hair and is a little big for his age, which means he can get into R-rated movies. He hates history and loves bacon, rats, and playing tricks on his squeamish older sister. When he opts out of a family weekend trip, he never guesses that he is about to take a terrifying journey into darkness. Hungry demons and howling werewolves haunt his waking nightmares... and threaten his life.

Presumably the only witness to the horrific and bloody murder of his entire family, a teenage boy must outwit not only the mental health professionals determined to cure his delusion, but also the demonic forces only he can see.