On vision, it's not necessarily a question of acuity, which is primarily what is checked in normal vision exams. Other vision factors include divergence/convergence/teaming and tracking. Also, it may not have been a problem 12 months ago (after all, he wasn't resisting reading as severely as you describe now, at that time); he is young enough that significant functional changes as the visual system progresses wouldn't be surprising.
His iReady results do tend to support the idea that his reading is not typical. Especially in the primary grades (through grade 3), a test like iReady is heavy on phonemic awareness and naked decoding skills (that is, not read in context), as deficits in these areas are critical predictors for reading failure. That tends to back up the possibility that he is not truly decoding (using phonetic reading strategies to sound out words), but is word calling (whole word sight reading) instead. You've been reading to him since he was little, so he probably has a great oral vocabulary base, and has seen many words read aloud, which makes it more likely that a child with good memory could have powered his way through memorizing a fairly extensive reading vocabulary visually. But this would likely be more effective for large, complex words, which often have unique shapes, than for small utility words, with their indistinct or nonessential meanings and common forms. And at some point, even the best rote memory runs out of space for storing all of these arbitrary images. Plus, retrieval of those pictograms is inefficient when many of them aren't tagged morphologically, semantically, or orthographically.
One quick and dirty test is to write down some isolated (not in a sentence) pseudowords, and see if he can sound them out. (I.e., words that follow the rules of English spelling, but are not real words.) If he has phonics skills, he should be able to sound them out easily. If he is predominantly a sight reader, he will have difficulty (either in accuracy or in speed) with more complex phonograms. (I used to pull out an old King James Version Bible and have them try out some of the names from the Old Testament, to see what they came up with.)
