Most boys that age are more into reading nonfiction than fiction -- dinsoaurs, cars, sports, how to draw, anything gross (lots of cool books out there for kids that are like this!).
Those that are reading fiction tend to go for the Diary of a Wimpy Kid series, Captain Underpants series, Goosebumps series (or any other scary book). You can also try Magic Schoolbus and Arthur chapter books as they are familiar characters from picture books, just expanded stories in chapter book format. I've even had boys read some of the Junie B. Jones series books (some of them are absolutely hysterical!).
As someone else said, find out what he's interested in and go for that area first. It's not so important what he's reading, as it is that he's reading! As always, take a peek at the book first to see if you think he can read it and it's appropriate for him in your eyes as you are the one who knows your child and his abilities the best.
To check for readability, you can use the 5 finger method. Have him read the first paragraph. For each unfamiliar word that he doesn't know, put up a finger. If you get to 5 before the end of the paragraph, find another book that's a bit easier.
And, finally, don't stop reading aloud to him. Kids love to be read to, (even the 6th graders) and it is a wonderful way to share time with your child and build their vocabularies.
Good luck & have fun!!
