I am not an expert, but have you looked into stealth dyslexia? From my (in-expert) understanding dyslexia is, in most people, primarly an auditory/phonemic problem. It's about the inability to hear the phonemes and map them with symbols. And it seems MUCH harder to pick up in very gifted child, particularly a younger child.
From the little bit we know about your son from reading this thread I am guessing that his speech development and reading development issues are directly linked and professional awareness of the severity is being confounded by his cognitive ability.
My eldest is not at all at your DS level of giftedness, and she has other issues going on, though we are struggling to pin point what exactly. But retrospectively I am certain that dyslexia, CAPD and being at least mildly, probably moderately, gifted have combined to present a deeply confusing picture.
At 7 yrs old her language and general knowledge was advanced for her age, but after 2 years in school we were still struggling with recognition of both the sound and name of each letter. Forget reading. Two and half yrs later she has the word recognition of a 12.5 yr old and even higher reading comprehension. When she gives a verbal presentation that she wrote herself, her delivery is well above average for her peers. When she reads aloud from a book it's like listening to sound salad. With great effort I can pick out individual words and tell that she is in fact decoding correctly and I can tell that she comprehends and even enjoys what she's read - but I can't understand a word of it because I am so busy trying to just figure out what each words actually is. I feel like I am hearing what she must have been experiencing all this time and I am astonished she ever learned to read at all, let alone make 7yrs reading progress in 2.5 yrs.
My own dyslexia was picked up by accident in adulthood when my compensation strategies were so solid that it was irrelevant to me to know, though it explained a lot. My DH was never diagnosed but I am sure he ALSO has dyslexia, but of a different form, his sister absolutely does. I think my eldest DD has inherited BOTH of our quirks. The book "Disorganised Children" has a really interesting section on Dyslexia that talks about the idea that there are multiple kinds of dyslexia, presumably of differing genetic origin, thus explaining the varying presentations, varying ways of successfully remediating, varying degrees of severity - and that you might get more than one "type". This was the great "A-ha" moment for me thinking about my DD and our family tree.
One thing I will say is that it was only when painstakingly teaching my eldest to read that I learned any phonetics at all. I remember being taught the e on the end of a word is silent (but not that it changes the vowel sound) and I remember the rule "i before e, except after c". And that is it. When I had a child at school and people started talking about "sight words" I remember being SO confused and thinking "Yeah but there are SO few rules and nearly every word breaks them so aren't ALL words sight words?". Um, apparently not! I was a late reader, but once I had learned the entire language as sight words, or enough of it to map the few words left I didn't know somewhat easily then my literacy took off. Actually a lot like my DDs - I was in remedial english in yr3 and top of my grade by yr5. And I didn't ever hit a brick wall, not with reading anyway.
The point of my story is not to suggest you don't need to figure out what is going on, or suggest that what happened to me was a great outcome. But just that in actual fact he might not ever hit that brick wall you are worried about using the memorisation approach (assuming you mean memorising words), at least with reading. Left to his own devices he may well learn to read completely differently to other people and still take off and have no problems once his own personal map is fully developed. But if he was my kid I would be digging until I had answers.
There are some posters on this board who know WAY more about dyslexia than me and have shared some really awesome links to articles on current theories of dyslexia. So try doing a search here for dyslexia.
