Have you thought about trying some sort of e-reader with him? One of the nice advantages to e-readers is the ability to change the size of the font making it a lot easier for people who have a problem with tiny font. And yes schedule a vision follow up it could well be a vision problem.
Absolutely try this. My DS started reading chapter books at 4 by himself only after we got the ebooks on kindle for him - we taught him to zoom in on the words first and then read the book as a precaution because we did not want him to get eye strain. And he read the whole Magic TreeHouse series on the kindle that way. The other advantage is that the pages are less overwhelming with fewer "black text" when the child zooms in. Now, he does not zoom in to the same extent as before, but we still remind him to do it to make it easy on his eyes.
You could call your VT to check if what he says about the letters getting smaller is an issue or not. To me it sounds like he is straining to make his eyes work together.
