Hi', Alan.
My lads are 42 and 45 years old now, but I taught them how to cast when they were about 10. I also used to run a one-day course for beginners on behalf of our angling club, and have fished with young teenagers at odd times during the 1980s and 1990s. The most important thing that I learned from all that was that to keep kids interested, you have to find a way of getting them into fish. If that means coarse fish, then so be it; although I didn't start that way wih my own lads.
They will respond to playing fish that you hook for them, but they soon tire of that, if they are really interested, and they then want to cast out and hook their own. Kevin is right about the weather, they will put up with a difficult day a lot better if it's a warm one.

Another good idea is not to give them too short a rod to start with. They need a bit of length to keep the fly and the line up off the ground. Cheers, and good luck with your lad. Terry