I've been trying out a number of readers and I think that Cool Reader is my preferred one now on my Nooks. I like the large number of font sizes that are available, and the amount of fonts. I like that I was able to customize my background color to a very pale green-grey color that is easier on my eyes than stark white. I like the one-tap switch to night mode. It also has controls that will stay on screen so they are easy to access. It allows for a great deal of customization, but you should be able to walk him through it because there will only be a few things to adjust for him.
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