I'm biased, but I think your son will love it.
There's a lot of complexity if you want to dive deep, but it's easy to have lots of fun. It's just a few clicks to create a star, planet, and moon system. My goal has been to create something that's accessible,easy, and fun to use. I'm not done, but I think I'm off to a good start.
There's a handful of easy to follow tutorials that are built in and I'll be releasing a few more later this month (May 2011).
A core contingent of the pre-Steam release forum regulars are 10, 11, and 12 years old. This is software made for both kids, teens, and adults.