I would be leery of letting your 5yo read too many "fat" books unless you read them all yourself first. My DS7 is reading at a 7th grade level on testing, but we've stalled out on 4th-5th grade fiction books due to content.
I haven't read Harry Potter myself, but my Mom did and loved it so I asked her and she said perhaps the first book would be okay for him, but it starts to get scarier and darker from there.
I was super excited when he began reading fat chapter books to share all my childhood faves with him, like Heinlein and Lackey, but I quickly found that most of these books I looked back on as innocent fare were too intense for him. I thought some of my older faves like Secret Garden would work, but he got lost in the slightly archaic phrasing, vocabulary and vernacular.
