One thing you'll need to start training some one is patience. It's easier to take something away from them and fix it yourself but you have to bite your tongue and watch if you want them to become competent.

A good memory also helps in remembering that we all asked a few stupid/obvious questions when we first started out.

You probably have some old machines and parts in your shop. Let him play around with that stuff and see what he can build. You need to have a good familiarity with the parts if you're going to troubleshoot them.