8 sure if young for a player. Hope he's unusually responsible and conscientious for his age.
The adapter in the video looks like it was homemade by drilling and tapping a spent CO2 cartidge. This works, but isn't particularly safe, and I certainly would be very wary of letting an 8-year-old play with something potentially that fragile. They make purpose-built 12-gram dummy adapters, but all of the ones I've seen are tapped on the side rather than at the end. That doesn't mean they don't exist though.
A better solution would be to track down one of the old Talon constant air adapters. Brass Eagle put these out themselves, and while they would do exactly what you need they're getting harder to find.
Honestly, for the amount of money it would probably take to set the gun up the way you want you would be better off just buying a cheap used direct-feed Nelson pump. I've got a couple for sale myself.
Finally, learning to know anticipate when a gun is about to run out of CO2 is a good skill for a young player to develop, especially if ever gets into stock play.