At that age, I had a very specific and focused Aspie interest, so even though my parents wanted to and did control my reading, it was something of a hard decision to make for them, because on one hand, ploughing through these fairly esoteric and wordy books at age eight- fantastic, lots of vocab, at that age it really does soak in easily- but it wasn't great subject matter (the Holocaust). Then again, although I was fascinated, I didn't always get it, it didn't sink in. I think you can count on the nature of an average 8 year old's maturity to shield them from a lot of that kind of thing. Swear words can be discussed- steer clear of the more explicit books, obviously, but access to that kind of genres can be more easily restricted anyway.
When I grew out of that, I moved on to the classics like Wuthering Heights etc. You could try him on those- some really beautiful passages. Again, a lot of it will go right over his head, but in terms of language acquisition, breadth of reading, those sorts are great. I reread and reread my old Enid Blytons for comfort reads. I also didn't consider my Babysitters Club books a guilty pleasure!
Adrian Mole is the kind of thing you can read whether or not you're clued in. I practically was Adrian at 14 and I took him very seriously :o but I still enjoyed it!
If he is motivated and has high concentration skills- start him on the Russians, why not. No long and intricate sentences to trip you up, very clear, again enjoyable without thinking about any hidden meanings- try short ones- maybe some of Chekov's funnier short stories. If he's willing, why not go for gold and set him loose on War and Peace.
Whenever someone asks for a book recommendation, I will invariably put Chaim Potok forward- I think he's fantastic- and novels like "Davita's Harp" and "In the Beginning" or "My Name is Asher Lev" might interest him because the narrators are children for a good deal of the way through (in fact, gifted children, which could also be interesting for him).
Or what about those Penguin 'adventure' or 'travelling' reissued books- the very thin books with the khaki covers, written by the original adventurers?
At that age, it doesn't really matter how great comprehension is- the important thing is to keep him reading. If he's bored with a self-chosen book, make sure he perseveres. Bring him to a higgeldy-piggeldy and spend a while in there just talking about books and let him choose a few.
There's nothing as lovely as rereading a book you read during your childhood, all nostalgic but discovering new things, see how much your perceptions have changed...