If he had dyslexia, that's an issue independent of test-taking ability. Doing poorly on tests was a side-effect of a more important problem.
For people who spend too much time on a individual problem, and they're told not to do that, but still do.....that's called idiocy, not bad test-taking. IF YOU KNOW THE CONTENT, AND DON'T HAVE LARGER ISSUES, YOU DO WELL. Period.