Alex Wise vs. the end of the world / Terry J. Benton-Walker.

Enhanced descriptions from Syndetics:

Welcome to the summer of the apocalypse. One 12-year-old boy leads the charge against the forces of evil as he tries to stop the Four Horsemen from taking over the world in the start to a wildly funny and addictive fantasy series about accepting yourself and finding your inner hero.

Alex Wise feels like his world is ending. His best friend, Loren, is leaving town for the summer, his former friend and maybe sort of crush Sky hasn't spoken to him since he ditched Alex on first day of sixth grade, and now his mom is sending him and his annoying younger sister, Mags, on a cruise with the dad who abandoned them. And, as if things couldn't get worse, a creepy shadow monster may or may not be stalking him.

But none of this could prepare Alex for the actual end of the world. Too bad that is exactly what's coming, after the definitely-real Shadow Man kidnaps Mags and she is possessed by the ancient spirit of Death-one of the Four Horsemen of the Apocalypse. Luckily (depending on who you ask), Alex is possessed as well by a powerful god who imbues Alex with their powers in an effort to stop the Horsemen...if he can figure out how to use them. So begins an epic battle between good and evil- Alex, Loren, a grumpy demi-god, and Alex's fourth grade teacher vs. Death, Pestilence, Famine, War, and the waves of chaos and destruction they bring to LA and soon the rest of the globe. Just your average summer vacation.

Alex is more used to being left behind than leading the way, but now he's the only one who can save his sister-and the world. That is, if he can unlock his new powers and see himself as the hero he is.

"Never has the apocalypse been so fun!"
-Mark Oshiro, author of THE INSIDERS and co-author with Rick Riordan of THE SUN AND THE STAR- A NICO DI ANGELO ADVENTURE.