Going to start off by saying that I do not have kids, so you should probably take what I say with a grain or two of salt. However, I was an adolescent fairly recently, and I'm also a senior in college studying English education at the middle/high school level. Because of my degree program, and because of my age and the age of my younger sibling, I read an insane amount of young adult literature.
I second all the suggestions so far, with special shout-outs to the Percy Jackson books and the rest of Riordan's YA novels, the Kingdom Keepers series, and most especially the Wayside Schools books (which is the first series I recommend to reluctant readers).
Is the Dear America book series still around/available? If your son is interested in history, those would be an excellent choice. These books are historical fiction novels set as diaries written by kids in different historical periods, and they are great books for reluctant readers.
When I was 9, I loved Anthony Horowitz's Alex Rider novels. I also enjoyed the novels of Margaret Peterson Haddix, including but not limited to the Shadow Children series.
One series that I have not seen recommended is the "The Secrets of the Immortal Nicholas Flamel" series by Michael Scott, which starts with The Alchymest. This is a great series that I cannot recommend highly enough.
One thing I would strongly suggest if you think your son is going to continue reading, and yay! if he does, is to find a good half-price bookstore in your area. In my experience, libraries often have limited copies of books, and waiting lists get insanely long (I've been on a waiting list for the first 39 Clues book for over a year). Having a good half-price bookstore in your area, or a trusty website to turn to if it's not at the bookstore (I use
Amazon for inexpensive books since I have Prime, or
www.alibris.com if I can get the book at an even better price), can be the difference between your son continuing to read because he can get the books he likes and your son choosing not to read because he can't get any of the books he wants. Often, paperbacks and a lot of YA novels can be bought at half-price bookstores or from online bookstores for significantly lower than list costs.
Hope this helps!