Vicki,
Was your son copying the conversations word for word, or was he just including some dialogue in the narration? The reason I ask is because as the ladies have been saying, narration is a personal thing and is meant to be a reflection of what the child drew out from the reading. So, while you may be wanting a summary of the reading (unless it is specifically assigned as a summary narration), there are many different types of narrations and all are acceptable.
One thing that you do not want to do is to give him the impression that a written narration must be done a certain way. It will leave him even more uncertain and keep him trying to write the way you perceive that a narration should be done. This is no different than writing responses to please the teacher in the classroom and is something we definitely want to get away from doing in the home setting (especially when doing written narrations)!
So, my first caution to you is to not make your son feel like he is doing it wrong!

Accept his written responses. Be sure to go over the Written Narration Tips (Teacher's List) in the Appendix. Then, go over his list right behind that. These
really help in doing written narrations and will give him that sense of purpose you feel he is missing. Make sure to also read "A Few Notes on the Transition to Written Narrations" in the Appendix as well.
Next, understand that his written narrations may vary quite a bit from week to week. For example, one week he may summarize more, the next week he may go into detail relating just one event that really struck him from the reading, the next week he may give you a detailed description of a person from the reading including dialogue, the next week he may interject some opinion within the narration. All of these are acceptable!
You can help him a bit by telling him to think about narrating as telling back a movie that he has seen to someone who has never seen it. Think how he would go about doing that and then apply that same strategy to narrating. Honestly, the more he orally narrates, the more shape his written narrations will take. It isn't unusual when being new to written narrations to "try on" various styles and ways of doing it. This is what writers do, and it is how they eventually find their own style.
So, I would encourage you to read over the Written Narration Tips (Teacher's List) and the lists that follow in the Appendix. This will help give you some guidance. Then, share the movie retelling example and let him write. Withhold judgment as to the narration's content, but do follow the Written Narration Skills list in the Appendix to help him edit the narration.
As time passes, you will see improvement. But, if you make this a teacher-pleaser assignment with one right way you are seeking, he will not come into his own as a narrator because you will have changed the assignment's original intention.
So, head to the Appendix right away for much needed help! It is there to encourage you in this endeavor!
Blessings,
Carrie