This guy is putting on his white overcoat and trying to look like a scientist, but he doesn't actually cite any real research in favour of his claim, and instead uses anecdotal evidence ("Usually, once the original condition is found and treated, the ADHD symptoms go away" and such) rather than statistics to support his conclusion and sell his book.

This isn't to say he's incorrect in his conclusion, he just isn't supporting it very well. I would hope the book has referenced stats but this article isn't very promising. He's not too far removed from a pilot claiming his story about a flying saucer must true because he was in the airforce.

For the record, I've been diagnosed with ADHD and have plenty of anecdotal evidence that it's for real, but may be suffering from confirmation bias.