One that I always return to on my shelf although it will be out of his depth at the moment but a good one for one he gets going is 'The Soaring Pilot's Manual' by Ken Stewart. Get him it when he's gone solo.
Nooooo... Get it for him NOW! If he's got more than half a brain cell that is - its really going to show him just how exciting it is, how much there is to learn and to master.
Please don't treat him like a child - this should be his opportunity to get into grown-up stuff. Some of the best soaring pilots I know are teen-agers and they are treated the same as every body else at the club - and they blossom as a result.