I will roll the dice with my answer. I'm sorry if it seems trite or dismissive of your concerns, I'm well intentioned!
Alright, thinking back to the fun I had thrown in the deep end with passed-down ex-work computers from my father: IBM 8086 and the MS-DOS 3.1 manual, and later on a 286 running Lotus Notes and some nifty File Explorer thing which I cannot recall the name of now--I was a bit older than yours, but I'm trying to adapt!--I would:
- throw the little tiker in the deep end (to some extent) and give him something he can figure out (and thereby take a feeling of ownership, mastery, and achievement over-- that is what you'd like to pass on and instill, essentially, yes?). this was my more practical suggestion. next I'll hit you with my slightly more zany, out-there one...
- get him a (cursed) old IBM ThinkPad or Toshiba. rock solid. Can take a beating. Got some miles on it. And old. Like so old that he has to use older software. Hear me out. I think that, if you plan a whole succession of machines over time to upgrade him in to (when the time is right), you can basically give him a "re-experience" of the development of tech, over time. Ideally, you'd be able to select a single machine that you could (depending on your inclinations and 'pain tolerance' perhaps), "expand" with him overtime by tinkering with the hardware to upgrade it to successive generations. Tho that may be overly-ambitious (for anyone!) and more trouble than it's worth. But I think there's good to be had in seeing the natural progression (Same way that a good physics or math course will take you through the derivations and historical context).
Anyway, I'm probably overthinking this, and I'm sure you have a bunch of your own ideas that are way better than this anyway, but just wanted contribute my little share in the hope that it helps you and he! :)