The boy who killed demons : a novel / Dave Zeltserman.

Enhanced descriptions from Syndetics:

So begins the latest novel by horror master Dave Zeltserman. The setting is quiet Newton, Massachussetts, where nothing ever happens. Nothing, that is, until two months after Henry Dudlow's 13th birthday, when his neighbor, Mr. Hanley, suddenly starts to look . . . different. While everyone else sees a balding man with a beer belly, Henry suddenly sees a nasty, bilious, rage-filled demon. Once Henry catches onto the real Mr. Hanley, he starts to see demons all around him, and his boring, adolescent life is transformed. There's no more time for friends or sports or the lovely Sally Freeman--instead Henry must work his way through ancient texts and hunt down the demons before they steal any more innocent children. And if hunting demons is hard at any age, it's borderline impossible when your parents are on your case, and your grades are getting worse, and you can't tell anyone about your chosen mission. A very scary novel written with verve and flashes of great humor, The Boy Who Killed Demons is Dave Zeltserman's most accomplished and entertaining horror novel yet.