I think there was a giant gap between expectations and reality. He wanted to program so he could make video games, but learning to program starts out text-based. The gulf between learning to code and learning to make video games like the ones he enjoyed is overwhelming.
I think the calculator eliminated that dissonance between expectation and possibility.
This is why I think javascript is secretly the best programming language to start someone out in as you can very quickly get to doing "cool" things - throw up a barebone html page with a canvas element and boom, you have full control over a graphical user interface. You figure out how to draw a rectangle, move a rectangle, and keep track of where that rectangle is, and you basically have everything you need to write a (very) basic game engine. With guidance, something like pong might be only a few hours away.
I think the calculator eliminated that dissonance between expectation and possibility.