Angry Birds is always good for keeping them busy. All of the different games (I think there are six now) are available in a free version that gives you the first 5 or 10 levels. And they all walk you through the motions at the start -- it's really easy to learn, and progressively challenging. Not particularly educational, though.
And if he (or you) gets addicted, the full versions are cheap, too -- I've gotten some (depending on device) for 99 cents and some for $5.99.
There are a couple of old threads on apps, though mostly Apple-centric. Many of them may be available for android, as well, however. Worth checking out!
Educational apps for iPad iPod/iPad apps loved by you and yours 
