Hey, I asked my resident husband-expert for you (he is a voracious reader of this kind of stuff):
"Shadows of the Apt - Adrian Tchaikovsky, I'd highly recommend this series to YA readers, and I enjoy the crap out of them as a 30 year old as well! Fantasy series about humans that have magic based on insects.
Codex Alera - Jim Butcher, Butcher was bet that he couldn't write a series fantasy series based on the concept of the lost roman legion, and pokemon. Well, here's that series! A good bridge between YA and adult fantasy.
Mistborn - Brandon Sanderson, another great fantasy. Sanderson has a great mind for logical magic systems, which will be enjoyable for him to wrap his head around.
The Farseer - Robin Hobb, Classic fantasy series, coming of age story. I can't see a kid, or adult, not loving it.
Prince of Nothing - R Scott Bakker, philosophically bent fantasy. A little higher level, but the concepts will make him feel smart and superior, which will probably push him to read the entire series!
Snow Crash - Neal Stephenson, classic cyberpunk that ages well. Can't recommend this enough for a young reader as well.
Chronicles of Amber - Roger Zelazny, I know you think Zelasny is old, and possibly too high level, but it's actually pretty low level compared to other books these days. Always a classic!
This is for the father but also for the kid:
Revelation Space series - Alastair Reynolds, the Mass Effect series was probably based off of the concept in these books
For the father:
Uplift series - David Brin, a little past the books you've read, but modern enough
Old Man's War - John Scalzi, in the same vein as Starship Troopers, but modern.
There's a lot more out there, and I've mixed in some more difficult reads, but overall, this should keep him busy until he can pick up the heavy hitters. "
edit: more..