I vote for Harry Potter.
The first one is easy, and they grow harder. But if he gets hooked, he'll puch himself.
Often boys prefer non-fiction. Try the national geographic books, if he is at all interested in science. My Grandmother taught remedial high school english, and she caused quite a bit of controversy back in the day 'cause she taught all the motorheads to read with popular mechanics. If he likes cars, get him reading about them. If he likes music, get him Rolling Stone and Spin. If he likes comedy, get him Gordan Korman or something. If he likes computers, get him on these forums.
The most important thing is to get him in the habit of reading what interests him. Once he gets the habit, he will push himself.