I never suggested that someone could walk onto set of any union show. But all the things I mentioned are non-union and it is possible to volunteer your way onto those sets with zero in. Now will he break into the industry doing a one week stint on a short or well funded student film no. But he will get an idea how a set is like and maybe make a connection or two. Many a spec short and student film have union members working them for zero to little pay? Why? To build their reel or gain experience. A camera assistant who wouldn't get hired to be DP on their studio day set would totally shoot something small to be able to build their reel to be able to bump up. I have no earthly idea how it works for an actor but for below the line crew that's how it works.
A week is only going to give a taste but better than stargawking to help your son decide if it's for him or not. Not a definitive answer but he'll know that that week times 100 is what it takes to crew in this business. Hopefully he finds a goid gig but even if its bad that's good experience because that's what it's like.
Oh I would look on mandy.com and craigslist crew gigs. I also look at cinematography.com job boards but that is a smaller board but arguably better quality gigs. But dunno if your son's interest is in camera.
I will say that this deal requires initiative and love of the art to stay in. For me I had the former but didn't want it enough to deal with all the downsides. Your son sounds like he has the initiative if he's giving college kids a run for it as a high school kid. Time will tell about love part.