The good news is, other than the reading comp score, which the neuropsych feels is a low estimate, his other achievement scores are very much in line with those predicted by his FSIQ. So underachievement does not appear to be a significant issue at this point.
On the second exceptionality side of the discussion, the behavior reported during the reading comp task could be consistent with the dysregulations in attention and behavior that presumably underlie your current rule-out diagnoses. The subtest is only the second task administered for most students, and appears to have been the first one for your DC (since the earlier subtests aren't reported). One would not expect him to be more fatigued for that task than for all of the later achievement subtests. Also, he did the best out of all of the subtests on the very next task. Lower reading comp scores are not unusual when attention is compromised, whether because of ADHD, emotional interference, or other factors; IOW, this might be a real result.
It may be worth examining the idea that both instructional mismatch and a separate dysregulation of attention are factors in his school experience. If either or both are documented, there may be some value in approaching the school regarding a 504 accommodation plan (say, for documented anxiety, depression, or ADHD) or GIEP. In both cases, some accommodations that might help include:
1. Reduced workload/item sufficient to demonstrate mastery (this means he only has to do as many items as necessary to demonstrate that he understands how to do them--good both for ADHD and GT, both of whom have reduced accuracy of performance when required to do a great deal of low-cognitive-demand repetition; the school may find this odd, given his high processing speed, but it really has to do with excessive repetition, not necessarily the need for extra time).
2. Alternate assignments to demonstrate mastery of standards (mainly so he can make choices--this is helpful for a number of concerns, including GT, ADHD, anxiety, and depression, as it allows GT and ADHD individuals to select higher-interest tasks from those available, and provides individuals with anxiety and depression to exert some additional control over part of their environment).
3. Curricular adjustment to instructional level. I would suggest, as we've discussed elsewhere, using the school's own curriculum materials to find his correct placement, ideally using a built-in placement test, or by pretesting through unit/chapter assessments until he falls below a cut score (such as 70% or 80%). Particularly in mathematics. His reading placement will be affected by written output, including some maturity factors, which may make identifying that placement a little more complex.
You may wish to investigate other supportive resources for high-cognitive learners. DYS, while excellent, is not the only organization out there. You may also wish to look into CTY (
https://cty.jhu.edu/), which serves students down to current 2nd graders. They have online and summer academic and enrichment programs, and decades of experience with HG+ children. Some families have found Mensa to be helpful. Your state has an active G&T organization, that may have additional resources or advice specific to your state.
There is also a poster (NJgifted) on this forum who runs a GT afterschooling center. (I have no direct experience with the center, but it may be worth you checking it out.)
