Edit: Here's something that came out recently (2013): Go: A Kidd's Guide to Graphic Design by Chip Kidd. It gets very positive reviews which tend to say that, while it's aimed at kids, the content is strong enough that it's good for adults too (I've not read it but thought it worth mentioning).
Maybe a design magazine at the lighter end of the market would be a better place to start? I don't know what's available in your region but I find Digital Arts, Computer Arts and Digital Artist all to be good, broad-ranging in focus and very accessible.
(make sure it's a magazine like those I link to that are for people who do design, illustration and digital arts, not a magazine for design connoisseurs who talk about design fashions. If it contains tutorials and hardware reviews, the adverts are for things like stock photo sites, design software, hardware and web hosting, and it has a CD stuck to the cover or a downloads section for things like fonts, stock photos and tutorials, it's for designers. If the ads are for expensive fashionable stuff and you're not even sure what the articles are about, it's for the connoisseurs).
I suggest these for these reasons:
- They have a mix of everything - examples of impressive, interesting and inspiring work, up and coming trends, practical tutorials to expand knowledge of software, interviews with designers, trade secrets. So it should give him ideas, boost his skills and also give him a clearer idea of whether this is a world he relates to and wants to be part of.
- They usually cover a range of styles and trades, whereas books usually focus on a particular niche or style in more detail than would be interesting to a newcomer to the field. This way he can get a flavour of what's out there and figure out things like what styles he likes, what designers to follow and what books to buy himself at his own pace
- It's more casual and less pushy. It sounds like he enjoys the experimentation and freedom, playing around and learning at his own pace. This is probably better for that.
- Their focus is on actually making things. They have a studio-desk audience: people who work or dabble in the field, people who love to create things. They have an interest in being accessible, inclusive, fun, open-minded. Design books, and the more up-market design magazines, however, often also have another eye on a coffee-table audience: this includes design buyers, academics, connoisseurs... They often have an interest in making design seem exclusive, inaccessible, precious, aloof.
- They usually come with a CD or downloads of handy resources like royalty-free stock photos and free fonts. For a kid who can make whatever he wants, about whatever he wants, in whatever style he wants, these can be useful fuel for experimentation and ideas.
You might want to check first that it's suitable for a 12 year old though... they sometimes feature work about moderately adult themes (probably nothing worse than he'll see on TV).
Definitely don't get him a beginner's book focussed on a particular piece of software if he's a fast learner who has no problem learning for himself. Those are usually paced for adult learners (i.e. slow). I imagine it would take all the fun out of it. It certainly would have done for me.
(oh, and if, 4-6 years from now, he definitely wants to be a designer and is serious about it as a career plan, or for anyone else reading this question with an older and more career-set wannabe designer in mind, get them a copy of the latest edition of How to be a graphic designer without losing your soul. It's a really good guide on how to get started in the industry without getting burned out or stuck in a dead-end, how to find what inspires you, and how to get there. But it's only suitable for someone who is actually starting to make serious, immediate moves towards a career. Your cousin-nephew has the blissful freedom and great advantage of being able to experiment and find their own passions and preferences at their own pace. It's a beautiful thing and should be allowed to develop naturally)