Ah. Well, there are a number of fun little kits out there, but the book I think my son would most recommend is 123 Robotics Experiments for the Evil Genius. It's pretty step-by-step. It's inexpensive used, parts are easily found, and it covers all your bases for learning to build your own bots.
My son was born with a soldering iron in his hand, so I can't recommend any kind of pre-built robotics stuff. There are organizations in our area that offer one-off sessions with Lego Mindstorms to a small group, though. You get to play with the stuff and you don't have to invest in a kit before you have experience with it. Perhaps there's a Bits Bytes and Bots in your area.