I would recommend Go. It would be easier to learn, and it's syntactically more similar to established languages like C, C#, Java, and the like, so it would be a good introduction to them.
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.