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One approach would be to record your story-telling sessions, particularly in a way that captures his reactions. You could then review those recordings to see what worked and what didn't (e.g. you ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/5242 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
One approach would be to record your story-telling sessions, particularly in a way that captures his reactions. You could then review those recordings to see what worked and what didn't (e.g. you had to repeat something in a different way). Reading comprehension is different from aural comprehension, but at least you know he'll know the words in one context.