Ask him if the words on the page are moving around or if words are overlapping or look superimposed on each other. When I tutored reading, that's what one third grade kid said and when I put a colored overlay (blue or yellow, didn't seem to matter with him) on top of his reading passage, he sped up a lot. I was timing him with and without the overlay on multiple days. Not a scientific experiment but I thought it was somewhat convincing evidence. DS's OT also mentioned this when DS had double vision after a brain injury. DS didn't get anything out of it but this other kid said that it helped.
I also think it's normal for kids to have tracking problems when they are young. Many of the kids I worked with (even second-third graders) had tracking problems when I first started working with them and they improved as they became better readers.
