I am convinced that anyone can learn to fix shit, I train apprentices to design and build shit and am in the lifelong process of training 3 kids and their spouses to fix shit. If I can learn it, anyone can.
but I started small. my first axle repair was on a Schwinn and when it failed no one got hurt (very much) and no one told me I was a failure so I went on to fix more and more axles, probably the most involved being an old VW bus with gear reduction boxes or a vintage tractor with a low speed split.
I would offer this small advice: Encourage and enable him, and try to learn alongside him (if I can, anyone can) and maybe the next grrrr will go better and cheaper.