Beer has dismissed the Tenori-On before as just a "toy", which is fine, he's used one and didn't like it so he's entitled to his opinion. Obviously, he didn't click with it.
I've used a Tenori-On too, though, and I agree with those who are recommending it. The price is obviously a big, big drawback. It may simply be unrealistic to spend that much money for what you're trying to do, and there's no counter-argument if that's the case.
Other than the price, the Tenori-On is a lot of fun to use and can be very inspiring, even for an adult musician. I like it because I can set it down on a table outside of my studio and just randomly pick it up and play with it for 15 minutes at a time whenever inspiration strikes. Do this often enough and soon you will have a bunch of song ideas to work with. I like this much better than having to go into the studio and power everything up before being able to do anything, it's like being a writer and having a notebook beside your bed that you can scribble down your random late night thoughts into.
Above all though it is fun and simple to use, so the chances a kid will enjoy it enough to keep using it are pretty good.
It's also very easy to record the results and then make something more with them. I'm doing just that right now and it is going fast and is fun, in contrast to other projects I've worked on that were a lot more time consuming and stressful. Some of the early demos are up here
Art Kerns on MySpace Music - Free Streaming MP3s, Pictures & Music Videos if you're interested in hearing what it sounds like. I will admit the Tenori has a rather limited sound palette, and more limited sampling ability than I'd like. Limitations can be a good thing though, especially when you're just starting out.
I also love the Electribe EMX-1 as an all in one sketchpad. If that was too complicated for him maybe you could still use it, but help by giving him basic patterns to start with, or pre-programming the synths for him, or something along those lines. Having him focus on only one function at a time (sequencing, synthesis, effects, etc) might make things easier.
Whatever you decide it's very cool that you are doing this! I wish I'd had similar encouragement, as it was I got piano lessons, quit them quickly, and then didn't start making music for fifteen years...