Sculptris is nice, free and basically like playing with clay. UE4 has a ton of content examples, and is free as well, a kid should have fun looking at the included levels seeing how they are made.
I would avoid programs like Moi or Rhino, if he starts with NURBS modeling it might be harder to adjust to the much more commonly used subdivision polygon modeling later. If you want to go the cheap or free route, you can give Wings3D a go. There's also Sketchup, which is lightweight, pretty simple to use (and you can…