I too like Sheffield and Clarke.
The best Clarke is before Cradle, though Light of Other Days is very good. Childshood End and The City and the Stars are IMO his best.
He could check out Strugatsky, "Roadside Picnic" and "Hard to be a God" are generally considered the best.
Also Robert Charles Wilson. "Spin" is maybe his best. "Darwinia" and "Mysterium" are excellent. I also like "Bridge of Years."
All these books paint remarkable, realistic worlds with characters of great depth. And they are not boring or 2D like so much that is being written.
And Iain M. Banks. "Consider Phlebas" and "The Algebraist" are my faves, but the latter takes 200 pages to really get started
It's realistically set on a gas giant planet. The best space opera there is.
Also "The Three Body Problem" is a good read. Can't remember the author's name.