If you can find them cheap, get short story collections by Arthur C. Clarke and Issac Asimov. Excellent for a young fellow.
do you know what, forget ebooks. Go to a used book store and look for the above there.
And if you can, find the original Robert E. Howard. In fact, I can do better than that: Give him this:
https://www.mobileread.com/forums/showthread.php?t=10446
Robert E Howard, The Tower of the elephant. An amazing short story. If the Robert E. Howard stuff is free (and I'm surprised it is), give him the lot. The original Conan stories are brilliant and entirely appropriate for a young fellow.
Edit: Good news. Tons more original Conan here:
http://ia700508.us.archive.org/21/it...obertE.Howard/
Rogues in the House, Red Nails and The Tower of the Elephant are the best. You'll have to some reformatting.