It definitely sounds like he might enjoy a CAD program. Most CAD programs have pretty steep learning curves. They're very difficult to just dive right in to, and figure out what you're doing. A CAD/CAM/drafting class at the local community college might be fun for him.
Try using sketchup first. It's a little more user friendly than the high horsepower stuff like Siemens NX. It used to be distributed by Google for free, but it looks like it's independently distributed now. With some digging, you should be able to find a free version of sketchup (maybe even the old Google version). There are TONS of videos on youtube, and articles full of instructions on using sketchup to create different things. If he likes that, a CAD class may be in order.
CAD is what all the pros use to see things work before they actually build them
Sounds like you have a future engineer on your hands