I am not sure how to answer. Part of me thinks starting out with Elixir would wire the kids’s brain to think about transformation of immutable data, isolating side effects, implicit return, and everything is an expression before encountering Imperative/OO programming. Would learning Elixir now be good for his long-term development and future career?
If he wants to make a game, which is the reason I picked up programming when I was 13, I strongly recommend giving him Godot and having him learn GDScript, which is basically a dialect of Python 3. It would be enjoyable for him, and he'd have something tangible, fun, and hands-on.
I'd probably introduce someone to SonicPi after something like Scratch. That way they can make some music and have some fun while learning some kind of Ruby. It's built to be educational so should be straight forward to pick up following the tutorials and lesson plans.
Elixir is a fine language really. I don't think you're wrong in anyway, it probably would teach some useful skills while also being less frustrating to learn than something like C, but I'm not sure there's much point in a child learning a language for the sake of it. They're also less likely to enjoy it.