All sorts of things come to mind reading your post so bear with me! I have a former-reluctant reader who was reading easy phonics books at 4 1/2 but had no interest in doing more. He's now 7 1/2.
1) Ditto the get his eyes checked post! My DS began wearing glasses just before he turned 5. His reading level jumped 3 grade levels in two months. Maybe it was the glasses, maybe his brain was finally ready. We'll never know- but he does have a severe astigmatism and insists on his glasses now to read anything.
2) Drop reading all together for a few weeks. Leave high-interest reading material all around the house. Set aside time for you to have quiet reading time, but don't read to him- just read for you. Don't mention it or put any pressure on him, just watch and observe. Does he model you? Pick up a book and start then put it down? Beg for you to read to him? You'll learn a lot about what's going on in his head if you just lower all expectations and watch.
3) Take him to the bookstore/library and ask him to pick 3 books- a too easy book, a medium book and a too hard book. Then take them home. Ask him to alternate pages/paragraphs in the too hard book. Make a fuss when you're done about how the too hard book isn't too hard anymore!
These are all specific strategies that people gave me when I was trying to figure out why my kid who tested HG+ wasn't reading. I was worrying about 2E issues, dyslexia and a host of other stuff.
Also, if you actually have a kindergarten teacher that will test the highest range of his level- you're very very lucky! It's pretty rare to get one that will even test beyond 2nd grade level. My DS is in 3rd grade now and I don't think he's had a full above level reading test in two years... other than the Lexile component of NWEA testing!
