Others who have more experience with this with their children may expand on this, but some families have found that evaluation by a developmental optometrist may identify deficits in visual tracking and convergence, as distinct from visual acuity, which is what a standard eye exam usually checks.
There are a number of families on here who have children with DCD (developmental coordination disorder), which also has some overlap with the behaviors and testing profile you report.
If vision of any kind doesn't appear to be a factor, then the OT direction might uncover fine-motor issues that can be worked on with therapy. If you had asked him to complete the 56 addition problems orally, would you have seen a different result? The focusing issues reported in martial arts and homework might actually be related to motor challenges, as those are both heavily motor-involved activities. OTOH, sometimes difficulties with sustained attention are the cause of slow performance.
These are questions to investigate with the original evaluator, and possibly with additional evaluations (fine-motor with an OT, vision with a developmental opt, attention with a psychologist, esp. a neuropsychologist, or with a neurologist).
