I was thinking CS Lewis already, good idea.
The problem is, I don't know which of his books to get!
Perhaps a "mere Christian" is too heavy reading, so something else would be better.
I believe the SPACE TRILOGY would be best for C.S. Lewis and a 15 year old male.
Chronicles would be seen as too childish. The other stuff too heavy an intellectual tome.
OUT OF THE SILENT PLANET
PERELANDRIA SP?
THAT HIDEOUS STRENGTH
That may be the three titles in the SPACE trilogy by C.S. Lewis. That's what I'd suggest.
THX.