Tha Narnia books are nice, although as is often the case with a series, the later ones become a little belabored. But as a series, certainly not at the same level with LOTR books or the Dune series.
At this age many boys develop interest in fantasy and science fiction. So, depending on how advanced he is, over the next year or two, he may enjoy:
The Golden Compass (
http://www.amazon.com/Golden-Compass...p/B000FC1ICM); The Subtle Knife (
http://www.amazon.com/Subtle-Knife-D...d_sim_kinc_1); and The Amber Spyglas (
http://www.amazon.com/Amber-Spyglass...=pd_sim_kinc_1)
Dune (
http://www.amazon.com/Dune-40th-Anni...9903874&sr=1-3) - the first 4 books are great
Lord of the Rings (not sure it's available as a legal ebook, but a good, immersive read)
Any more accessible science fiction.
I'd second Oz and the Lost World though.