It sounds like he is a simultaneous processor, rather than a sequential processor--often found in visual-spatial kids, which kids on the spectrum often are. I would guess that he has memorized the majority of his reading vocabulary by sight. I would further suspect that he has not automatized decoding by phonemes or morphemes, which is why he can sound out when pressed to (and when you scaffold the sequential aspect of it), but doesn't use phonetic or morphemic decoding as his go-to. It's just so much more work for him than memorizing sight vocabulary. He may be doing better with writing than reading phonetically because the spelling process forces him to go through the phonemes/phonograms sequentially; he probably doesn't have the same breadth of memorized spelling vocabulary as he does of reading vocabulary. I notice you mention that he makes reversals on words with which he is familiar--where he does not have to go through the sounding out process--which would support that hypothesis.
I would be inclined toward investigation with regard to a learning disability, such as dyslexia/dysgraphia (the latter because I would predict that spelling will emerge as a greater concern as the expectations for spelling accuracy and fluency go up). With the ASD Dx, at some point, he will probably need support for inferential reading comprehension and writing associated with literary analysis.
