I'd agree with the above. I read voraciously at that age - anything was good, children's, teens, adult, whatever - and I doubt it harmed me any. Some stuff was boring, so I just skipped those bits or those books.
Of course there was less explicitly "adult" stuff around when I was that age, and I'd agree there are things that are possibly "too adult" for a child of six, but basically anything that is aimed at / categorised as middle-school or younger YA range shouldn't be harmful. Whether it interests him or not is another matter, but action/adventure considered suitable for middle grade / 9-12 age group should likely be very fine.
Old classics such as Treasure Island (or lots of stuff by Jules Verne, depending on the quality of translations) should be fine, too, I'd think.
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