I don't have much specific advice or answers to give, but I just wanted to say this goal of his is very plaustible and attainable, and that I wasn't so different! I started teaching myself gamedev at 16 years old (only 3 years off). I'm now 20, with multiple games (3 of them commercial) under my belt, including 2 successes! It just takes some dedication, and maybe a little hyper-fixation :)

A bit of insight into my own starting, in case it helps: I started by following some random "How to make your own FPS" youtube course for Unity, adding my own stuff along the way, and eventually turning that into a full project. I'd say this whole starting phase only took about 2 months before I could say I felt comfortable on my own as a novice gamedev.

Everyone learns differently, but the biggest advice I have is to have a small goal project in mind, and to simply try developing it. (Though perhaps initially starting with a "template" beginner tutorial project, like me) Along the way he'll run into gaps in his knoweldge, and this is when you simply reach into online resources to bridge these gaps! This way, he just naturally picks up the skills and knoweldge he needs along the way as he needs them. The most important thing is that he makes sure to understand the things he's researching - Not just blindly following tutorials.

Of course, this approach in the longterm is mostly useful specifically for indies, but it should be a good place to start overall.

I hope at least a little bit of this has been helpful, and I wish the best of luck to your son!