I started programming at 8 or 9 years old with QBasic on an old computer. My dad just showed me how to do one or two simple programs (simple loops), but I learned pretty much everything by myself with the included documentation.
I know QBasic is a really shitty language for real projects, but it's fun for a kid because it has a lot of basic functionalities included, he can easily do simple I/O, draw graphics, etc. A few years later I moved to php, and then to C and C++. I've forgotten pretty much everything about basic, but I know how to code.
So just give him a QBasic environment, QB64 provides an IDE that looks and behaves like the original QBasic/QuickBasic IDE, and runs on modern hardware/software. It should be perfect. (Though I don't know how good the documentation is)