Thank you, everyone!
I will ask his OT about visual tracking. I suggested he use an index card below the line so he doesn't get lost, but he refused.
I think the font, color illustrations and type of paper do have something to do with it. He doesn't like newsprint books like Magic Treehouse. He read Nate the Great fluently and with feeling, making different voices for all the characters in 15 minutes straight, then had a complete meltdown at the end saying he hates to read and can't do it. Turns out he hadn't eaten lunch that day, but it was weird - and sort of frightening.
I think it's partly self-confidence for him. He reads beautifully when he agrees to do it. Maybe he'll be like CFK's son and he'll just be fluent all of a sudden. Problem is that I'm nervous he won't read at all. But in more rational moments, I realize he loves books, has always loved books, and will learn to read. Right?
Thanks for all the great hints. EandCmom, I will pm you. We have a fabulous OT, and I'll speak to her about the visual tracking. As I recall, it wasn't a problem in either of her evaluations, in kindergarten or first grade.
I downloaded Reading Blaster the other night, which he enjoys. He has no problem reading those books because there are not many words on the page. (And level 1 and 2 are simple.)
Oh, and when we did the evaluation last year, I also brought him to a pediatric ophthamologist (where's spell check when I need it?!!) and she said his vision was fine.