I just have a few “brain storming” questions and hopefully others will chime in with some thoughts and suggestions.
Have you noticed if it matters if he reads the books to himself, or if he listens to the book being read?
Does it seem like a choice, for example, he expresses that he does not care for a subject or topic and then you notice that that is what he struggles with narrating?
Does the subject matter ever come up later and you realize that yes he did retain the information, just struggled with putting into words at the time? For example a few days later he mentions that something he heard or read reminds him of something from a previous reading.