Need ideas for encouraging budding animator

My son, 12yo, is showing an interest in, and some talent for, computer
animation. Like a lot of kids, he enjoys comic strips and classic
comics and also draws his own comics (his writing is better than his
artwork). He's also familiar with a lot of styles of cartoons, from
Rocky & Bullwinkle and Wallace & Gromit up to popular kids' cartoons
(though we do limit TV watching). He astounded me recently by using
PowerPoint (which happens to be loaded on his computer, a Win98
hand-me-down) to create a several-minute-long, reasonably complex
animation. I know PowerPoint isn't the right tool for the job, but I
don't know what is. I'd like to get him some software suitable for
beginners that will introduce him to principles of animation and give
him the tools to do some fun stuff. It would have to be something
that's been around a few years, so it would work on his computer, but
then I'd also be happy to get something used. I just don't know what to
look for. Something designed to teach animation techniques would be
perfect. It would be great if it were directed to kids, but I'm not
looking for one of those branded software packages that let you insert,
say, Disney characters into pre-designed backgrounds.

I'd also be interested in recommendations on books or other resources
for him. This is a field completely alien to me -- I have art/design
training, but it was all before computers!

I'd be willing to send him to a summer class if I can find a suitable
one in this area, but summer's a long way off, and in any case he'd
probably do better if he first has the chance to learn some of the
basic programs/tools, whatever they are.

Suggestions on how to encourage this interest are very welcome.

--Robin