I would suggest that, since his spelling and thus writing currently are compromised, and because of the apparent achievement gap, it would be helpful to him to be evaluated for stealth dyslexia (also known as compensated dyslexia), by a school-based or private psychologist (neuro, school, or clinical). If this is his learning profile, then it is likely that reading/spelling interventions based on the Orton-Gillingham method would help him, and may allow him to show more of his ability in written expression. In the schools, either OG, or the closely related Wilson, are the gold standard for remediation. Even though he is a very effective reader at this point, it would help his spelling. You could also, if he is willing to work at home, try the OG-based home program, All About Spelling, which is fairly affordable, and designed for an untrained parent or other tutor to use.
www.allaboutlearning.com. Barton is another home or tutor-provided program, though somewhat pricier than AAS. There are also private OG and Wilson-certified tutors, often speech language pathologists, special education teachers, or reading specialists.
If he has not been identified with a reading disability by the school, which is not uncommon for those with the profile we are hypothesizing, he is probably not receiving the appropriate OG-type reading/spelling intervention. If he has not received individual or very small group reading support, then he has almost certainly not received it.
You may or may not find the school receptive to evaluating and qualifying him with dyslexia or specific learning disability in reading/writing. Either way, there are other paths to supporting his educational success, as many parents on this board can attest.
