Don't buy him anything yet - you can refer him to the free Wally Kahn ebook collection of (usually older) gliding books on the Lakes Gliding Club website.
Wally Kahn eBook Collection
Wally's book "A Glider Pilot Bold" has some priceless sections in it
(The two assemblers of this collection are sadly no longer with us - Pete Redshaw died a year ago and Wally last week - but may there be lots of gin and only very small amounts of tonic wherever they are.)