If you're trying to keep it light, he might be inspired by the game-like approach of Alcumus, the on-line problems on the AoPS website. They don't have the same challenge level as the textbook questions, but they cover all the material. And you can jump around to any topic you like (in pre-algebra or elsewhere), or just follow the logical progression. There are videos for the whole book there too, which may help if he gets stuck.
And while it might seem a bit ridiculous, there's nothing that says you can't print the Alcumus questions to work out on efofex!