Hard cases don't get backpacked. And they'd seriously hurt your back if you tried, I think. The only time I've ever seen anything like that is for cellos.
However, there are companies who make extremely well done and well padded (the most important thing!) gigbags. Warwick, for example, offers what I think is called the Rockbag. It's a great product for around $40 (about 1/3 of a hardshell case) and weight about 1/4 of a hardshell. You can put it on your back or carry it on the side. As far as guitars are concerned, I'd go with a gig bag.
A good padded gigbag is actually more protective than a hardshell in most cases simply because of the padding, which hardshells lack. While you can put weight on a hardshell, unless your son intends to stack stuff on top of it or go on tour, it's unnecessary. Gigbags protect against common wear and tear like bumping into walls or falling down stairs, being dropped on the ground, because the padding absorbs shock and therefore cuts down on a large amount of the damage.
Hope that helps, Grenalda.
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