I've always made sure my kids were bored. When they get too into one thing and narrow their interests, I know it's time for some time to figure things out on their own. We did this from early on. It really helps the imagination kick in. If necessary it can be kick started with a new toy or change of location-- sandbox, playground, swimming pool. But in general, it sounds like your DS just doesn't know how to "find something to do" on his own.
I think that some of the problem arises when the "find something to do" is always reading, watching TV/movies, or computer games.
