"The Complete Stories, Vol. 1" by Isaac Asimov (Easily the best collection of short stories I have ever read)
"Tales from the White Hart" by Arthur C. Clarke (A simple, yet entertaining collection of short stories)
"The City and the Stars" by Arthur C. Clarke (Main character is a young man)
"Ender's Game" by Orson Scott Card (A bit of violence, but still a very good book)
"Ender's Shadow" by Orson Scott Card (Again, some violence, but I would still recommend it)
"The Time Machine" by H.G. Wells (A classic, not very long, but very good)
"Halo: The Fall of Reach" by Eric Nylund (Especially if they like the video game)
"The Boxes" by William Sleator (I loved this book when I was a kid)
"Marco's Millions" by William Sleator (Another Sleator book I loved when I was young)
"A Wrinkle in Time" by Madeleine L'Engle (I read it when I was in elementary school, and even though I didn't understand much of it, I still really liked it.)