One thing I forgot to mention about the books - if you look at the spine of each one, at the top it will read "Year 1" "Year 2" etc... and that will also help you remember in which order to read them. If you tried to go out of order you'd end up horribly confused because each book in the series builds upon the previous ones.
As for the age appropriateness of the books themselves, I think that's a great question that's very difficult to answer. As the series progresses it does get much darker and there are things that happened that bothered me to a certain extent, of course we all know that I am incredibly sensitive

lmao: ) but still... each child is different and I would hate to tell you that it's OK for Ellis since I really don't know what kinds of things might upset or scare him.
I do think with the first two books you will be fine, and probably even the third book won't present much of a problem. But by the end of book four there are some pretty disturbing things that take place - I think you might be better off reading them yourself first and then deciding what you think Ellis can handle.
I bought the first 6 books for my neice last summer when she was 8, but I made sure my SIL read the last two books in the series first because I was really concerned about the impact some of the character deaths and a few other scenes might have on her. I guess it shows that I'm not a parent because my SIL read it all, never flinched, and then handed the books off to my neice who read them without turning a hair.
Me...I read Half Blood Prince (which was great, by the way) and bawled like a baby during the last two chapters.
But like I said...I'm much more sensitive about these things than most people...
I do think that by reading the books to him you will be better able to handle any fears or concerns that he might have. I think he'll really enjoy them - my neighbor was reading them to her son a few years ago when book 5 was released (I think he was about 8 at the time) and he just loved it. My neighbor told me that she and her son would talk about each chapter and try to figure out what would happen next - they only did one chapter per night to drag out the suspense.
I couldn't do that - I pick up the book at midnight when it's released and stay up all night reading to finish it. Then I take a nap and re-read so that by the end of the weekend I've already gone through it twice.
I'm a geek, but I'm comfortable with that.