How about Prince Caspian or one of the other characters from the Narnia books?
My 10 year old is really into the Alex Rider books - that would be pretty easy to do (he's a teenage spy working for MI5). He also likes the H.i.V.E. books, I can't remember the main guys name, but he is like the opposite to Alex Rider. The other books he has recently gone through are the skulduggery pleasant books - A skeleton wearing a mac and hat and scarf is the main character.
As it is meant to be your sons favourite character, he'd better get reading!
Oh another one, He could go as one of the pirates from the How to train your Dragon books.