That's maybe the best way to drive a 12 year old _AWAY_ from software development. What's really interesting is building stuff and seeing how it runs. When working alone you don't need to design with models/diagrams (heck, you probably don't even need that in a team most of the time). You don't need unit tests or a deployment strategy or a schedule. Just build stuff, have fun and try to learn something.
I know this does not help the original poster. Most books about iPhone/Android development are probably not suited for a 12 year old. Maybe starting from tutorials or little open source apps is a good way: Read the code, try to improve it, see what works. Iterate.
That's why I said "concepts" (not diploma), it's up to him how in-depth and/or hands-on he wants to go. But learning "how app's stuffs work" (the components of the app - design, code, controls/interactions, data) is the foundation on "building" one (that's because I don't believe a one-thingy app an app, like putting a photo on the screen, it gotta be 2 or more stuffs assembled together for me). If he just look inside an app, and he does not know what's the meaning of the "code" (module/library, object), programming, rebuilding/iterating, etc., he won't learn how and why it was built like that. Heck, he would't have any idea what he's looking at! Fundamental concepts, no matter how layman you can teach it to the kid, and having a "clear idea/plan/purpose" are essential before you send him to "just build stuff".
For a kid, drawing boxes of things he would like to put into his app, and seeing how he's going to put them together, is the best way to start an app, and the best way to identify the concepts/skills he needs to learn.
To get you started, I suggest you (or the kid's parents) tell him to draw how his app will look like, all screens (from the home screen to the scoreboard :)) on individual boxes, the size of his iPad/mobile device, then you guys cut them out. Then you link these screens/cut-outs with the navigation controls, for him to understand the flow, and the "big picture" of what's he's building.
These are of course, if you want him to build "something," not just practice/unfinished works that do nothing -- therefore does not contribute to the kid.
(I taught basic Photoshop to kids, 8-12 years old. They were able to compose cool graphics because they understood how to make bitmaps, shapes, texts, layers, blending and effects work together. I didn't underestimate them, they needed to know what the heck they're doing, so I really taught them those things, in the end, we had an unbelievable exhibit! :))
I know this does not help the original poster. Most books about iPhone/Android development are probably not suited for a 12 year old. Maybe starting from tutorials or little open source apps is a good way: Read the code, try to improve it, see what works. Iterate.