I learned BASIC as a kid (first on a mainframe terminal with line numbers), and use Python today. I think the language is not as important as other issues:
1. Computers are so much more advanced now, that the obvious gap between beginner and commercial software is vast. This can be discouraging. When I learned BASIC, I could write programs that didn't look any more crappy than typical commercial software, even if there was a performance gap.
2. There's more magic involved today, meaning that when you run into an obstacle, you wave a wand and a library or stackoverflow post magically appears. To do something in the old BASIC, you had to figure out a way to compose it out of the 13 keywords that you had already learned. So you were forced to think about algorithms, right off the bat.
3. You were closer to the hardware, which was more exciting if you were interested in things. I could kinda explain what the computer was actually doing. I loved math as a kid, but I also loved electronics. The day that I programmed a thing to do something (maybe just a blinking LED) was like the rapture for me.
What's closest to the old experience for me is not a language, but a thing: A microcontroller board. Out of the box, it has no GUI, so no expectations for what "cool" software looks like. And you can make it do things that have an immediate sensory effect. You can now put CircuitPython on a Teensy 4.0 board, and sidestep having to learn C, at least for a while. And your son can really own the thing that he's programming.
As a minor suggestion, you can also get closer to the computer and to programming by making more use of the command terminal when showing your son how to do things.
A final word, don't be heartbroken if it doesn't click. It might not. This is also something parents are told when their kids start music lessons.