Just for some variety, Apologetics Press has some readers that I was planning on purchasing for my soon-to-be 9yo. He reads at about the same level as your son. One is the Early Reader set and the others is Learn to read. Here is the website:
http://www.apologeticspress.org/catalog/index.php/cPath/2_5/sort/3a/page/1
I also found, at our library a set of books called Extreme Readers and another titled We Both Read. There are both silly stories in this last (which we skipped) and nature stories (which we liked).
Also the Nature Readers from Christian Liberty are good, and their Robinson Crusoe Reader (I think they publish this one) has been a big hit with all my children at this level. One last is The Life of Washington. This is a great encouragement because it is written in one-syllable words yet has a very grown up ‘feel’ to it.
Blessings,
Cindy