Have you read the Narnia books yet? If not, they'd probably be a great choice. I read Cornelia Funke's Inkheart series to my DD just before she turned 6, and The Indian in the Cupboard and a couple of the sequels when she was 6. I really enjoyed Inkheart and the sequels myself. The Bunnicula books by James Howe have been big hits with my kids. DD was 6 when I read the first couple to her, and DS has enjoyed them even younger than that.
Some other series that might work for a 6 year old, but might be even better when he's a bit older:
the May Bird series by Jodi Lynn Anderson
The Mysterious Benedict Society series by Trenton Lee Stewart
the Silverwing series by Kenneth Oppel
the Swallows and Amazons series by Arthur Ransome
Some other books that are good, but not particularly long:
Peter Pan
The Silver Crown - Robert C. O'Brien
Mrs. Frisby and the Rats of NIMH - Robert C. O'Brien
Ronia the Robber's Daughter - Astrid Lindgren
Mio My Son - Astrid Lindgren
The Brothers Lionheart - Astrid Lindgren
Coraline - Neil Gaiman
The Graveyard Book - Neil Gaiman
The Princess and the Goblin - George MacDonald
Igraine the Brave - Cornelia Funke
Dragon Rider - Cornelia Funke
The Mouse and the Motorcycle - Beverly Cleary
Ginger Pye - Eleanor Estes
Mr. Popper's Penguins - Richard and Florence Atwater
Where the Mountain Meets the Moon - Grace Lin
It sounds like your kid likes things that are exciting and maybe a bit scary, so I included some that would be too scary for some 6 year olds. But I'm pretty sure all the books I listed are ones at least one of my kids listened to and enjoyed at 6 or younger.
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