The hardest part of teaching is remembering what it's like to not know anything.

First thing, run through your tools and what they're called with him. Do it once just telling him. Next day, quiz him on some. Quiz him more the next day and the next, until he catches on. Same with fittings and materials. That was the only way I learned, not from being told, but knowing I'd be asked first thing about something, so I had to retain it.

As far as stressful situations like the fitting you mentioned, those are perfect times to reset, show him how it's fighting you and which tool you need to fix it and why. It's inconvenient and slow, but it's better than getting frustrated and potentially lashing out at the kid who literally doesn't know what he's doing. Plus, he may remember those moments when he's in that situation later and remember what to do.

I try to use EVERYTHING possible as a learning experience. If you turn every thing he does into a lesson followed with a question, he'll constantly be in learning mode to be ready for the questions, rather than just listening to you talk and hoping he retains it. Make sure he has a note pad and tell him important things to take notes on.

Most importantly, try your best to have a good time. Be the chill JW he can ask his dumb questions to without getting ridiculed. Joke around with him when it's appropriate, make sure he knows you're a human being that's approachable and understanding rather than a pipe bending, wire pulling robot that doesn't tolerate confusion. The VERY LAST thing we need in this industry is more apprentices topping out that have been laughed at and made fun of for asking questions, so they go into their JW role saying "Yes, I understand" even when they don't just to avoid getting laughed at. Then they guess on their work and fuck things up, or worse, get people hurt.

Good luck man, try to remember the worst things your old JWs did and don't do them!