My son is finished with the Lightening Thief by Percy Jackson and the Rick Riordan series but needs something similar - he loves "realistic fantasy", Mystery, Adventure and Humorous fiction. Any suggestions would be greatly appreciated. Series are great because once he's hooked he's a voracious reader.

I assume he's already read Harry Potter?   Chronicles of Narnia is another classic.  

There's also Diane Duane's "Wizard" Series.  (First book is "So You Want to Be a Wizard'?)  

I was about that age when I read "A Wrinkle in Time" by Madeline L'Engle.  I couldnt' interest my son in it, but it is still one of my all time favorite books!

Depending on the child, he might also like Garth Nix's "Keys to the Kingdom"  but you might want to vet them first - they can be kind of dark.  The series is also not complete yet - the last one is scheduled for March 2010.

Not fantasy, but my son loves the "39 Clues" series.  It's scheduled to be 10 books, each by a different author.  Rick Riordan wrote the first one. 

Has he read the Hiccup Horrendous Haddock series by Cressida Cowell?  He might enjoy them.  They are about a young Viking boy who is destined to become the leader of the Hooligan tribe but he is rather small and bookish and doesn't really look like a mighty Viking warrior.  Add to that, his best friend, the bespectacled Fishlegs, and an undersized and toothless dragon and you have a set of adventure books that are hilarious.  My son (now eleven) would howl with laughter over these books.

Try the shadow children.  My child who enjoyed the lighting thief enjoyed this series. It is about a world were it is illeagal to have more than 2 children.  The series is all about 3rd children and how they want to be free.  It is a super series.  My one child one she started them she could not put them down. I am not doing justice to them. They are great. I read them with the other child and I really loved them as well.

I second the shadow children by Margaret Peterson Haddix - my 10 year old loved the series and also loved Percy Jackson. 

 

Another one he really enjoyed is the City of Ember Series by Jeanne duprau

He may also like the Artemis Fowl series by Eoin Colfer...

My picky son loved the Alex Rider series by Anthony Horowitz at that age.  It is about the many adventures of a 14-year-old orphaned boy spy.  The books have been compared to the James Bond movies.  The first book is Stormbreaker.  (I have it on my bookshelf if you are interested.)

My boys enjoyed the Prydain Chronicles by Lloyd Alexander (starts with "The Book of Three") and "Trapped in Death Cave" by Bill Wallace.

I liked the Charlie Bone series.

These are really helpful. I've got him on the 39 clues right now and the How to Train a Dragon series.

AMONG THE HIDDEN (SHADOW CHILDREN SERIES) BY HADDIX is a great choice for one who like the Lightning Thief!! He could try Frank Pereti or Ted Dekker....may be  a little scarier? Perhaps Bruce Coville books-- the Magic Shop series?

I recommend the Septimus heap series.  My son loved them the first one is called magck.

He might enjoy the Tunnels series by Roderick Gordon and Brian Williams.  The first three are out.  The fourth one will be released in May in the UK... no US release date yet.

Also, he may have already read them, but Roald Dahl books are classics.  Danny, The Champion of the World, The BFG, Matilda, Charlie and the Chocolate Factory, Boy, James and the Giant Peach, and The Witches are all great ones for that age.

I would suggest  The Magic Thief series by Sarah Prineas :  The Magic Thief;   The Magic Thief: Lost;   The Magic Thief: Found 

Not sure if you are still checking this thread but my 9 year who loved the Lightning Thief series just read Gregor The Overlander by Suzanne Collins and loved it.

"When Gregor falls through a laundry room grate in his apartment building, he plunges into a world where spiders, rats, and cockroaches co-exist with humans. This world is on the brink of war, and it seems that Gregor has a role to play in the Underland's uncertain future. The boy wants no part of it -- until he realizes that it may be the only way to solve the mystery of his father's disappearance."

 

Though not a series, he might like "Holes" by Lois Sachar. (I have an unpostable free with any order from my shelf. It's slightly water damaged and has some light staining.)

I also recommend the Septimus Heap series by Angie Sage. There is another series by Bruce Coville, the "My Teacher..." series. The first book is about students discovering that their teacher is an alien, and what they do to go about getting him to leave our planet. There are 4 books in the series. I think all are available for order on PBS. I still have my copies from when I was in 5th or 6th grade.

If your son likes Peter Pan, he might like the series written by Dave Barry and Ridley Pearson. They are huge books, probably about 400 or so pages. The first book, which is called "Peter and the Starcatchers" is an origin story for how Peter came to Neverland. There are a few books in the series that are smaller and are written for younger readers. Currently I think all these books are WL'd, but I got all my copies through Amazon and B&N online because they were bargain books. Most of them are out in paperback now.

Rick Riordan has a new series out call the Kane Chronicles.  The first book is The Red Pyramid.  It was released the beginning of this month.  I think the teacher who had it said that it was about Egyptian gods, but I'm not sure.