Don says that he blew through the first books. But the "most recent volume, Order of Phoenix, seems different from the previous four volumes. It goes deeper into Harry Potter's mind and tries to teach something to the kids. While I breezed through other volumes in matter of hours, I struggled over several days to read few chapters of the latest volume."
Hello? Earth to Don? What is wrong with teaching something to kids? You make it sound like a bad thing. Walk through any public place and look around. You'll notice that today's kids have *a lot* to learn.
Don also says "My son had similar problems so he skipped through most of the book. When I asked him his opinion about the book, he said it was fun. I didn't probe deeper because he was obviously faking it." I'd be very interested to know how old Don's son is. Maybe he's not old enough to be able to appreciate the struggles Harry is going through as he works through his teen years.
The books are written so as to follow Harry's travels through school. In the first book he's ten and in the fifth he's 15. His view of the world while young is much more shallow and superficial than it is as he grows older. Some people, and that includes Don, may want Harry to stay in his fantasy world where everything is black and white and the grown-ups know best. In my opinion, though, the series is nothing but better with the addition of the more realistic themes and Harry's more young-adult view of the world. The fifth book definitely didn't go south.
And there in lies the rub. I don't think the fifth book was particulary geared towards fifteen year olds. I'd guess that when your son is fifteen he may feel differently about the book.
And that's part of the problem with writing a series that follows someone as they grow older while at the same time trying to "keep it real". It would have been easy for Rowling to strip it down and make another light and shallow quick read. However, if you really have been bonding with the characters as the books have moved on, I think you expected more. I know I did. Again, I'm glad it turned out the way it did, as I enjoyed it and look forward to the next one. I'm sorry you didn't feel the same way, but hopefully the future installments will renew your faith. I sure am looking forward to them.