I hope this helps
http://www.goodreads.com/list/show/3349.Best_YA_Children_Series_Involving_Greek_Myths_
I absolutely ADORE goodreads.com
EDIT #1:
My DD13 (Avid Reader) loved the Percy Jackson series as well as many others. The only "Greek" one she could think of was
http://www.goodreads.com/book/show/2342943.Oh_My_Gods_ but it's really girly and more YA than Juvenille. But just an FYI for ya. The second book in the series looks cute too.
If you'd like recommendations on "quest" type books can get a ton of them her. Let me know. She says the "Erec Rex"
http://www.goodreads.com/author/show/3624.Kaza_Kingsley and "Leven Thumps"
http://www.goodreads.com/author/show/125984.Obert_Skye series might be a good place for a 9 yr old to start if he likes "quest" books like Percy Jackson - but they are not mythical in nature. She could only think of "mythical" books in which the myths were made up by the author and not so much based on greek mythology.
She has also read tons of medieval/dragon stuff as well. She's a great resource but she puts 5 stars on just about everything she reads so if you want specifics let me know and I'll ask her for you.
http://www.goodreads.com/review/list/2129382?page=1&shelf=read&per_page=200
EDIT #2:
If he's looking for more "facts" then maybe he'd like a non-fiction book of stories? Bailey liked the story or Perseus and Andromeda in
http://www.goodreads.com/book/show/79626.D_Aulaires_Book_of_Greek_Myths That, actually, might be your best bet.

Took me long enough.
Ahh... this sounds more like a "must read" list. I'll have to look it up.