I can't remember how old I was, but I have a 3 minute movie transferred to video showing my Dad letting me shoot his six shooter out in the desert. I think my Stepmom was taking the movie, it's black & white with no sound - the latest technology at the time.
I'M REALLY NOT THAT OLD!
Seriously, try this. Find any information you can on the SPORT of shooting. Specifically, the recent Biathalon contestants in the Olympics. Check around your local area for school teams or with the local Jaycees (they hold annual national shoots, I believe). Basically "arm" yourself with information on shooting that doesn't include "Lethal Weapon", Mel Gibson stuff. The NRA has the Eddie Eagle program (which doesn't even allow real guns to be shown), and they put out a Jr. Shooting Magazine. The idea is, your nephew is likely to hear about and possibly become exposed to firearms sometime in his childhood. His parents should be interested in his learning what firearms are all about and the safe way to handle or not handle them. Explain that a young boy's curiosity will often override any fear of parental discipline when noone is around to tell and/or peer pressure is at work.
When I was a preteen, I was at my Sister's home. The babysitter was in the livingroom with my two young nieces. I went poking around in my Sister & Brother-in-laws room and found a Colt Python .357 revolver...loaded! I pulled back the hammer just like they do on t.v. and then I got REALLY scared! Somehow I managed to safely lower the hammer back in place and put the pistol away. This kind of story isn't unique, but it can have deadly consequences. Haven't you heard of kid's shooting one another accidently with a found loaded gun? Ask the boy's mother if she knows, without a doubt, which houses have firearms and which don't. Children need to be familiar with firearms. Not only how to operate one, but they also need to see/feel how destructive these can be. This will impart a much better message than telling them to "not touch or go near any firearm". Talk is cheap.