Magic Tree House and Magic Schoolbus are popular, if you haven't tried them already. I personally found the MTH rather dry, bland, and formatted, but a lot kids seem to like it.
If your son is itching to try something a bit more advanced with language or imagery (or if you want to gently encourage or push him in that direction), some of Dahl's books (Enormous Crocodile, for instance) are great. I did this with my eg/pg son who's in kindergarten too.
Flat Stanley series, Nate the Great, Henry & Mudge (maybe too easy for him), Frog and Toad or advanced books by Arnold Lobel (maybe too easy for him), Amelia Bedelia, Fox or George and Martha (by Edward or James Marshall), Pinky and Rex, Little Bear, Mr Men and Little Miss (humor and language are more advanced) are others.
I think you just have to see what sticks, keep encouraging him to read, follow his lead, and his/your comfort level. There's also advanced picture books for reluctant readers like The Stinky Cheese Man and Other Stupid Fairy Tales or comic books like Captain Underpants (which are quite popular).
You could look at Amazon and see what they suggest or you could try Scholastic's wizard (
http://www.scholastic.com/bookwizard/). Scholastic gives you the reading level and suggestions for that level/age or above. There's also a lot of reading lists online for gifted kids if you get stuck.
