Don't buy him anything yet -
Such a moot point. If he is not into reading much, then it can be pointless shoving books under his nose. Now or ever. To get him enthused about gliding's story line, find the movie The Red and the Black. It is about two American gliders (1-26s) and their pilots waging a furious battle in the sky.
It would be interesting to hear any comment your boy had to make after viewing it.
There has to be a trade off. If you are going to facilitate his training, then part of the deal should be that he makes a concerted effort to study; study the theory and read some of the best things ever written about gliding.
You need to look astutely as to how he takes to his first dual sessions. After a few flights it should be obvious whether he is hooked. If not, do not flog the dead horse. The enthusiasm of the instructor is very important, especially at the beginning. As in all fields of endeavour, there are instructors and there are instructors. Having been around glider fields and gliding clubs, many of, over the years, I have seen the best and the worse. The latter may be technically highly competent, but sadly are unable to communicate any real enthusiasm. And if they do not relate particularly well to the younger generation, the poor pupil is on the back foot from day one.