That's really going to depend on his personal preferences. Science Fiction and Fantasy are really very diverse, with multiple sub-genres within them and they sometimes overlap.
For instance, I (female, mid-20's) tend to prefer fantasy, and within that genre I enjoy dark swords and sorcery fantasy. Books I've enjoyed recently include Joe Abercrombie's First Law Trilogy, Steven Erikson's Malazan series (10 big books), and Scott Lynch's Gentleman Bastards. Some not so recent favorite include R. Scott Bakker's Prince of Nothing trilogy, China Mieville's Perdido Street Station and associated books, and Matthew W. Stover's Heroes Die.
My boyfriend, on the other hand (mid-20's also) tends to prefer hard science fiction. His favorites include any of Iain M. Banks' books, and Dan Simmons' Hyperion Cantos, among others I can't remember off the top of my head.
That said, I had him read the Malazan books, which he loved and he has been excitedly waiting for the next book right along with me. He has had me read a few of Iain M. Banks' Culture books, which I loved. And we both independently loved Terry Pratchett's Discworld books (British humorous fantasy). There are plenty of other sub-genres, too, like urban fantasy, soft sci-fi, space opera, alternate history, etc.
Why not get a subscription to Fantasy and Science Fiction magazine?
http://www.sfsite.com/fsf/ I'm afraid they don't have an electronic version yet, but it's a great magazine with a wonderful diversity of stories. You could also try Analog or Asimov, one of which I believe has an electronic version, but I think both of those are more sci-fi oriented.