I have to ditto those who say get him into gliding... for two main reasons:
1) He can solo at 14 and get his ticket- a real pilot's certificate, limited to gliders- at 16.
2) It's generally cheaper and takes less time than the power-PP rating.
Also, if he goes that route, then decides to get into powered flying, aside from having a very solid foundation of basic skills to build on, he will already have a lot of the required book-learning out of the way (there is considerable overlap between the powered and non-powered study requirements).
I've heard that youngsters do very well with glider training - it is a challenge for sure, but it's just a bit more simple and a bit more limited, so it's not too "grown up" for the average teen. The completion-of-training an safety stats are very good in that age group. We have a 14-yr-old in our glider club who had a little powered training and general exposure (his Dad is a pilot/owner), and he soloed last season while I was still smoothing out the rough spots (I already had over 200 hrs and the PPASEL). He took to it like a fish to water, and I've been told that is not unusual. He had a blast, learned to be responsible, achieved a goal he'd been yearning for (to solo an aircraft) and stayed out of trouble all summer.
There are extremes of soaring, of course, but that stuff is not a requirement for the PP-glider.
I am biased a bit, of course, but for his first flight I heartily endorse a glider intro lesson.
If yo are going for the PPASEL, there's no reason he can't tag along on some of your lessons (assuming it's not in a 2-seater) while he's working on his PP-Glider. That would be a good way to expose him to that whole thing.
Another source of info:
http://www.ssa.org/
Aside from the glider question, I think it would be best to not make it seem like "homework"- get him a discovery flight in something (Young Eagles, perhaps), get him involved in any kind of youth-outreach program in your area that involves aviation. The serious study should wait until he's decided what he wants to do, IMHO. But it certainly won' do any harm if he just likes to read he books. Some of it is bound to sink in...