Has your son read "Holes" by Louis Sachar? Not a book series -- just a single book, but it's a fun, fast read, as are others Sachar has written. If he's a mature reader, Roald Dahl's books for younger readers (James and the Giant Peach, Matilda, the Witches, Charlie and the Chocolate Factory, etc.) were stories I enjoyed reading as a kid.
So many great options depending on his level. Some faves are Secrets of Droon Series, Box Car Kids and Hardy Boys Mysteries. On the higher reading level for that age, The Spiderwick Chronicles series is wonderful.