Have you tried explaining to him that the words are harder and that it's totally ok if he misses some words? I've had some success with this but a lot of it is just a personality trait and something that will continue to come up over the years. My son is like this as well and it is very frustrating as a parent. I am trying to teach him that it's ok to not succeed at something immediately and that hard work is effective, but it is not an easily learned lesson.