Alrighty! Here's an update how things are going after a few months. This is what happened:
I decided to borrow my Korg EMX-1 groovebox to my nephew. The logic behind it was that he already seemed to be interested in it and it has lots of interesting flashing buttons and knobs to tweak. It's very hands-on so he can pretty easily create his own drum beats with it. My sister says that the kid has been making beats with it quite often and has also developed an interest in his home keyboard which he previously didn't like at all!
That's a good sign!
I haven't heard what kind of noise he's currently making, so that's something I have to figure out soon. I don't believe he has learned how to create melodies/basslines yet. That's something I want to teach him by linking the groovebox to his home keyboard and showing him how to sequence external gear
After he learns that, I'll teach him the basics of Garageband. Hopefully I get that far before he gets bored with the thing...