I'll second what Emu said. While I learned on SolidWorks (With a very big company handling the bill for it of course), I can say Google Sketchup is as powerful with regards to modelling individual parts. Where SolidWorks justifies its cost is in assembly simulation, force distribution simulation, finite element analysis and other things that you'll not ever need unless you're actually designing something that needs to fly with passengers or survive a high-speed crash. I'm only just beginning to learn Blender myself and it's a whole different world, but I would reccomend downloading both types.

If Sketchup is graph paper and a ruler, then Blender is canvas and brushes. The best thing for your son is to help him figure out what kind of design he wants to do.