[Rbneader]

What age range does he typically read? That really impacts my suggestions.

Getting books for kids is tricky - some kids aren't prepared to handle some topics even if their peers are, and some are way ahead of their peers in certain areas. So YMMV on the age ranges below. Maybe check in with his parents about what he's had problems with in the past, or what they don't want him reading yet?

General suggestions:
Andre Norton has a lot of fantasy for different age ranges. Her Witch World series is great for middle school and up, and she also writes sci-fi that works for 10ish and up.

Also Diana Wynne Jones, Patricia C Wrede and Jane Yolen.

All of the above write books for both kids and lighter YA that works for 10ish and up, but is still a good read for adults. They also all have distinctive writing styles and worldbuilding, if that's what he's looking for.

Age-based suggestions:
I read The Hobbit in elementary school, so that's a suggestion.

In the definitely-for-kids range, E Nesbit and Edward Eager are pretty good.

If he's an older teenager, Martha Well's Books of the Raksura series are a lot of fun and very different from typical D&D style fantasy. They mention serious topics but in a lowkey, matter of fact way. I'd feel comfortable handing them to a teenager after a talk about how people live in fictional worlds doesn't necessarily reflect how things are in the real world.

If he likes lighter sci-fi, Timothy Zahn writes good adventure novels. I especially like the Blackcollar series and Icarus Hunt, the Cobra series is bad.

If he's an adult and prepared to confront serious topics in detail, Deed of Paksenarrion is amazing. But there is one attempted rape and one torture scene that get detailed and I wouldn't let a highschooler read them. Also Bujold's Vorkosigan series for space opera, but again definitely some scenes I wouldn't want a high schooler to read.