Use a box of the worms you buy as starter material for your new bin. Remember that the nightcrawlers like different kinds of bin treatments, and in particular do not like as much disturbance as wigglers can take. There are good descriptions of their care on line. I have been impressed with how quickly the nightcrawlers multiply when fed well, and not disturbed. The ones I have are quite a bit bigger than what is sold in stores: they appreciate all the food and care.
Renais

Can you just put two or three red wigglers per hook? This might deplete the bin some but 16 year olds need as many good years as they can get fishing before other things take them away from fishing. A local school yard after a rain has nightcrawlers free. Use a flashlight to spot them outside of the light. Then snap them up. Or so I've heard this is the way. No need to purchase this way. There should always be babies and cocoons in the bin. So even if you harvest every adult one summer to meet your son's fishing needs there should still be some worms that will fill in soon. If you start feeding them more also start giving them more bedding to keep their balance. Keeping a youngen fishing I think is a good use of worms. Maybe he would like a bin of his own to experiment with with a bit of starter worms from your bin. I'm into low or zero cost so any container would work. Maybe he will even catch some extra nightcrawlers and have some to sell to those fishing who have run out. I've read stories where big fish are caught with tiny bait. There are also lots of Youtubes about how to capture worms.

In my country the cheapest sources for worms are the vermicompost farms. They sell online and you can get the worms by the kilo (2 pounds) for 40 bucks. I guess they also exist in the US because everything marketable does exist in the US ;-)

And they also sell the vermicompost a hell of a lot cheaper than shops. The price of a bag of vermicompost has become my standard to know whether a shop makes honest margins or abuses the buyer ;-)