My 9 year old son, who reads on a 5th grade level loves the Percy Jackson series. Check out your local library's children's department. Even though ours was decimated by Hurricane Ike, our Galveston library has rebuilt their children's/young readers' section so well. There are lots of great books out. I read 90 Miles to Havana earlier this summer. It was a wonderful book.
Bluebonnet books or any other award winning book are a great start too. There is a reason they won an award.
Also, what is he interested in? Find books on that. My son loves anything Star Wars, he'll read anything Star Wars.
On a side note, my son had a horrible time learning to read and was retained in 1st grade because of his reading grade. He went to summer school, passed and moved on to 2nd grade. He will be a 4th grader this coming school year and is now reading a whole grade level ahead! I was so worried he would hate reading because of his troubles.
This past year his teacher made the class transition from picture books (though he does still read some) to reading chapter books. He had a hard time with it, but I told him the pictures were in the words, and he can take those words and use his imagination to paint the picture in his head.
His sister got him the first Percy Jackson book for Christmas. As he was reading it one night, I heard him struggling with some of the words. I was worried that he was concentrating so hard on the words, he wasn't going to remember what he was reading. So, I told him to put it down and that night we started a new thing. Before he goes to bed, he lays down on the couch and I read a chapter to him. He has to close his eyes and just listen to me read. We read the first Percy book and he was hooked. He's working through them all and reading more about mythology.
I agree with Brandy, above, my son loves the Diary of a Wimpy Kid books too. Main thing is get books on things he is interested in.