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This brings me back to 2001. I was 12 and subscribed to Computer Games (mag). There was some game on the cover, complete with an in-depth preview inside.
More importantly, there was a history of D&D prefacing the game. The game was NWN, and the edition at the time was 3.0
Reading through the history of D&D and simple explanation of play, I instantly fell in love.
But I was 12. I didn't know where to buy the books, much less have the money to spend $100 on core books... So I built my own system.
It started with maps - I got some graph paper from my dad, and initially would just imitate the Village of Hommlet map from the history article's first page. I drew more and more, linking the pages at the edges. I realized I could stack maps too, putting underground dungeons connected to the overworld by caves and buildings.
I started writing up a rules system too. Everything was based off the d6 (that's all I had), and I had classes, races, items, monsters; you name it.
I never finished it and I never got to play it with anyone, but I've been a system designer and a mapper ever since. Keep up playing with him, I'm sure he appreciates it in a big way.
Buy him some dice. Dice are amazing.
It sounds like he's got a good grasp on creating his own rules already, so maybe don't introduce him to some complex mini/RPG system just because - if he keeps discovering or inventing new mechanics on his own, bring in a game that uses them as an implemented demo, but let him keep exploring without constraining or pigeonholing his creativity.