My son is really taken with Kerbal Space Program. This great games allows you to design your own space craft and so run your own on-going space program, all with a realistic physics engine.
What is particularly nice is that this cross platform Mono based application is being built in a very agile manner with a new release most weeks, each adding features as well as bug fixes. There also seems to be an active community of people building plug-ins for extra space craft components and rovers.
I am not sure how much orbital mechanics will appear in his school exams this year, but it is certainly educational in the longer term.