Does your son have any vision problems at all?
The fact that he did so well on digit span makes ADHD seem unlikely, but I'm no expert. Coding and symbol search rely on quickly scanning images and if he has any vision problems or visual processing disorders that would make those tests very difficult for him. You mention his scores dropped on the timed achievement subtests, but he did exceptionally well on the block design cognitive test, and that's generally considered the most difficult timed test.
