What is happening every time you read that same story to him is that he is actually learning the story while you read. You will find after reading that same story to him for about the twentieth time that he is saying names and bits of the story as you go along. Repetitive reading will also help your child to widen his vocabulary.
This is his way of learning and as he learns to master the characters and names in the story, he will feel as if he is accomplishing something great - and of course he is! When you really are tired of reading the same story page for boring page, change the names of the characters or parts of the story line - your child will feel absolutely triumphant when he can correct you!