Whew! Thanks for all the detail!
Beginning from the easy stuff: I would not be at all surprised if there were a dysgraphic element involved, based on what you've said about fine motor tasks and spelling. Given that he does not spell well orally, either, it may be that there is also a dyslexic quality to this. The early reading profile you describe could well have resulted from memorizing words by sight, sufficient to make it through most easy readers and short chapter books. In the absence of efficient phonetic/morphological decoding skills, his reading probably comes to a screeching halt every time he encounters an unfamiliar word, that he has to decode, which is probably why he reads so slowly. Even so, with the exceptional working memory found on his cognitive testing, this would actually be a manageable way to read, up to a point. It may be that he has now reached that point. Your curriculum (prior to backing up) expected him to read at a level where decoding was not expected to be a task in itself, but a tool for comprehension. The combination of labor-intensive reading from memory, poor phonetic decoding, and higher-level comprehension may have tipped the balance in terms of reading being manageable anymore.
Based purely on what you've reported, I think this would be my best guess. Of course, for better data and diagnosis, I would suggest having a full evaluation, including achievement testing, beyond the cognitive testing you had done for school entry. It is also possible, as I've mentioned to a few other parents in related situations, that he's developed bad habits related to asynchrony, and that he simply needs to be re-taught phonetic decoding in a systematic way, at which time he will self-correct to the less labor-intensive approach to reading (if my hypothesis about his reading is correct). In that case, a few months with a systematic phonemic awareness curriculum (such as All About Reading, Logic of English, Lexia, or Reading Horizons) might be enough to get him back on track.
Harder stuff: Clearly, you are dealing with a bunch of other complexities, which can certainly affect concentration, mood, and compliance/motivation. For example, sometimes focusing on inhibiting tics leaves to little attention and mental energy for other tasks. Continuing to sort that out with his health care team definitely is important. It may be that you will be in the situation of making periodic changes for some time to come.
ADHD: the core deficits of ADHD are attentional dysregulation and poor inhibition (impulse control). It doesn't necessarily present as a short attention span; some have unusually long attention spans, but difficulty shifting appropriately. Again, for a better read on this, a comprehensive evaluation would make sense, since many different neurological or health conditions can manifest as they same behaviors. Kids have a relatively limited repertoire for communicating that "something's not working smoothly". So lots of things look like inattention, distractibility, hyperactivity, but may have very different causes.
Other: growth spurts can really make kids feel uncomfortable in their own skin, which can affect everything else. Thinking about the difficulty tracking, has that been investigated (visual tracking/convergence problems)?
