Is it only SF he likes, or does he like fantasy as well? Possibly the best books I've ever read are George R. R. Martins - 'A Song of Fire and Ice' (known as 'Game of Thrones' on the telly box). Truely Epic!
For Hard SF I've yet to read anything better than Ben Bova's Mars and Mars Returns.
Then there's the classics he's almost certainly already read... Dune series, Greg Bears 'Eon' series, Arthur C Clarks 'Odyssey' series.
If you can track a copy down Pier Anthony did a kids book called 'A Race of Time' which I thought was particularly good.
In the Sci-Fi/Fantasy vain, another good read (or at least first trilogy was) is Stephen Donaldsons 'The Chronicles of Thomas Covenant, the Unbeliever' series.
The Thrawn trilogy extends the Star Wars universe better than I'm expecting Disney to. I'm also partial to the odd William Shatner Star Trek tie in

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I tried Steven King's gunslinger series but after about three books I found that they were getting longer and more drawn out with each edition. I think I made to 5/7 and gave up!