You might start with a subscription to flying magazine, then, perhaps a membership in EAA, introduce him with young eagles.

When I was his age, I'd hide in the school library morgue, where piles of musty smelling copies of aviation magazines were stashed that had been donated and put away . I spent a lot of time in there reading. Not all of it made sense at first, but I learned through context. I found every flying book I could lay my hands on, many of them dating back to WWII and before, and read them. I got my first airplane ride after doing a boy scout aviation merit badge; the instructor took every scout flying who completed the badge. I built shelves of model airplanes, flying and plastic static display. No internet then, so it was all pages and reading.

Get him some plastic models to build, let him research what he's building. Take him to air museums, airshows. Let him feed his thirst. Don't force it on him, though...it's a quick way to burn a kid out, and there are a lot of other things competing for his attention. None of my kids were particularly interested in aviation, and I never forced them.

I had a private pilot manual as a kid that was so dog-earned, highlighted, and annotated, and I spent so much time dragging it everywhere with me, that my mother wrote "The Bible" on the cover.

Barry Schiff has a series of books that are excellent on flying topics that are of interest to a student or private pilot, as well as professional pilots. Called The Proficient Pilot, I highly recommend all three. It's not starter material, but for someone who's really interested, very good stuff.

Get him Sport Pilot as a subscription with his EAA membership. AOPA membership has Flight Training Magazine, also a great publication for students, or potential students.

Don't overlook soaring. Available at 14 years old, he's closer to flying a sailplane than a powered aircraft at the moment, and it's invaluable experience and understanding of flying the wing.