I've been writing JavaScript since I was about your son's age and am now one of the editors of the JavaScript specification. His code is much much better than mine was at his age!
I have one piece of feedback for him about the code itself, in case he's interested (of course the important part is the game, not the code, but I'm more qualified to give feedback on the JS): put `"use strict"` at the top of every <script>.
I have one piece of feedback for him about the code itself, in case he's interested (of course the important part is the game, not the code, but I'm more qualified to give feedback on the JS): put `"use strict"` at the top of every <script>.