The Chronicles of Prydain (
Wikipedia) by Lloyd Alexander is a simply wondeful classic epic high fantasy series loosely based on Welsh mythology and I highly recommend it if it seems compatible with what the parents would allow. It's got a few Newbery Medals and a Disney film adaptation, and there is no overt sexuality in it and even the one reference to sexual assault as a threat that I recall is very, very veiled. Also one of my personal favourites, which I still re-read from time to time.
Terry Pratchett's
Only You Can Save Mankind and the other books in the Johnny Maxwell trilogy are a bit more sci-fi IMHO, and you said that the kid prefers fantasy? But they're pretty good, and you could also try Pratchett's Discworld books for younger readers such as
The Amazing Maurice and His Educated Rodents and the Tiffany Aching series, which should mostly fit your requirements (there's a bit of innuendo in the latter due to the occasional presence of Nanny Ogg being, well, Nanny Ogg, but nothing overt).
Diana Wynne Jones has many kids/YA-suitable fantasy novels, and perhaps the best to start with might be
Howl's Moving Castle due to the possible appeal of the anime adaptation, and the Chrestomanci books are also very good, although both are tilted to a slightly younger audience, IMHO. My personal favourite is actually
Fire and Hemlock, but it's probably not a good general recommend because it's a bit of an experimental sort of storytelling type of novel.
More sfnal, but from a devout Christian author, Madeleine L'Engle's
A Wrinkle in Time and its associated sequels & spinoffs are also quite good and should surely pass muster with the parents, although they might not be to the kid's taste if he prefers epic quest type stuff.