Give 123D Design a try: http://www.123dapp.com/design It is free, and seems to have enough features to get your son used to using a CAD program. There are several other "clay modelling" programs which might better suit the way he thinks when he is creating objects. He can load 123D to a PC or his iPad, which is handy. He can do some design work while he is on the bus travelling between home and school, then refine his work when he gets home.

I use Rhino 5, but it is expensive and, for kids his generation, it would be "old school" to him.

Old Man Emu