When I was young, generating visual things was my gateway-drug to programming. With all the tooling, frameworks, workflows, concepts, etc. that you have to learn today in order to do even simple things, programming can become pretty overwhelming to someone who is just starting out. Processing is like a sandbox that is simple, but still keeps you close to the metal and provides a fun and liberating environment to grow your skills.
It may not be for everyone, which also depends on what your son is interested in building, but creative/artistic expression through code is something that I believe everyone should experience at some point.
There are many great learning resources for Processing that cover the whole spectrum from very easy stuff to more advanced projects that involve physics simulation, fractals, 3D graphics, etc. I especially recommend the video lectures by Daniel Shiffman, who teaches even advanced topics in a fun and engaging way: https://processing.org/tutorials
freeCodeCamp the website with its main curriculum is really good.
freeCodeCamp, the YT channel and republisher isn’t that good.
They hoarde all kinds of stuff from other people on YT, and not all of the courses have good quality. I have seen some awful explanation/teaching in some videos on the fCC YT channel.
Is there a good recommendation for teens who're just getting the programming bug?
My son's getting hooked but their curriculum is terrible (think Excel or word )...