It's not JUST mains.
Youngsters must get in their heads, there are tools to see this invisible stuff that BITES.
We already have the benefit of experience, your and me, but how do we effectively pass it on?
Some thoughts. No particular order.
Get or make a battery powered electric fence, a very simple one will look very innocent until you touch the wrong end. Old lawn mower ignition is a good experience, just a gentle rotation while holding the lead will do the trick.
Boys especially can be tactile learners, a harmless shock will make them think twice about where their fingers investigate.
My first shock was from a small engine ignition lead at ~10 yrs old.....45 yrs later and I still haven't forgotten.
Visual aids can be good too, set the electric fence to jump a tiny gap(after he's been shocked) then take him to an engineers to observe welding.
Warn him, if you ever find out he's been doing stupid stuff, don't come back.
Get him a DMM, even an old AVO, turn the process into a project, get manuals, demonstrate safe use, get Croc clips for the leads(safer for nervous hands).
Glass fuses are impressive when they blow, remember safety glasses.
End of rant.
Never had any problems with my 2 boys, their mother wacked their fingers if they went near power sockets. Every kid is different, you'll have work out what suits him.
Good luck.