I think some of the remediating mentioned is more like what we did with my DD. She had extremely high comprehension and very poor spelling. We went back and did a phonetic approach to spelling, with gives you the phonograms and the rules that apply, then moved to root word, prefix and suffix meaning. Often spelling is approached as a list of words to memorize and as one gets older if they don't have those rules it becomes more difficult to spell and decode, especially unknown words without context.
All of the scores you mentioned can also be impacted by processing and working memory issues, so it may be as simple as that. You just need to keep coming back to your last phrase or two, "acceleration to match his level of thinking". Only you know if the possible bottlenecks indicated by his scores could hinder an acceleration, but that might be the basis of concern you are hearing from others.
