He's already had enough of Scratch, and I don't want a Python graphics library, because it's time for him to add JavaScript to his toolbox (in addition to Python, not instead). He's very artistic (his Photoshop work is amazing), and we both want him to spend the upcoming summer using artistic projects as a vehicle for learning JavaScript programming.
I'll check out P5.js and Processing.js. Any other suggestions or ideas?
The home page for my personal site has a starfield made entirely with canvas: http://ivanish.ca — best to view in a dark room so you can see all the pseudo-nebulae.
Source code (if it helps): https://github.com/ivanreese/ivanish/blob/master/source/scri...
You might also want to consider Elm, but I have very little first-hand experience with it: http://elm-lang.org