The Bestbuy deal was great. Doubt we'll see much lower for similar item this season.
Most 12-year-olds will be suited for any current machine. But something less powerful might mean less time gaming and more time studying

Netbook is generally more efficient, and easier to stow in a bookbag. Battery life and size are important for people on-the-go.
Older people often dislike the small screen and keyboard. But for young hands and eyes, it could be a perfect fit.
Of course in a few years, he'll be much bigger and want an upgrade. At this point a used item might be a good choice. Computers are not good long-term investments. Needs and wants change with time.