Has he ever tried to read a book this way before? There are quite a few differences in screens. Some will be easier to read than others, on an individual basis. Also, does he want the ability to have the book read out loud to him? This is a nice feature to have when your eyes get tired; you can close your eyes and listen. However, with ebooks (in contrast to audiobooks), the voices are synthesized. Although the synthesized voices keep getting better, they are still not totally natural and can be annoying to some people.
Do you have a Best Buy near you? I would recommend going in, asking what options are available, and taking a look at each. This way you can at least rule out a few. They should have Kindles, etc. there for you to check out.
Edit: I suspect the best source for free eBooks would be your local library. I would go to the library and ask about it. That may actually be the best place for you to start. I don't think all libraries have this capability, but some in your state should (and you can usually arrange for a non-resident library card). See this article for more info:
http://blog.the-ebook-reader.com/201...ing-overdrive/