Earlier quoted context omitted.
Is this serious? I started programming around 9 or 10 on a Commodore 64. I would go to the local library to get programming books for it. Now you can hit F12 in your browser and get a powerful console into JS. JS is fine, encourage the thirst for knowledge.
JS is an objectively bad programming language. It is not fine, it is just annoyingly common and convenient. The main advances in its ecosystem are related to transpiling it from other, saner, languages. If he has fun writing JS, then great for him. He's 12, he'll do it if he wants. But it'd be doing him a disservice not to open his eyes to much wider, greener pastures of the programming world.
Back in the day we started with Qbasic not because it is a good language but because that was easy to set up, we all had it and our friends had it and our school computers had it.
And JS is less bad a language than Qbasic.