Sonya and Tristan both had good ideas. A couple of things from our experience - my dd 6.5 has also been somewhat of a reluctant narrator, but we are making progress...
- I don't know what kinds of things you are asking him to narrate, but yes, start with shorter/simpler things. We've been reading through Aesop's Fables this year specifically for narration practice and she generally does pretty well with these - better than some of our other books.
- I find that my dd gets overwhelmed by "tell me everything you remember" prompt and sort of freezes up. I've found wording things a little more narrowly helps her to avoid getting overwhelmed. Things like "What was the most interesting thing you learned about ____." or "What was your favorite part of the story?" Once we get past that I can ask her to elaborate on why and then I get more details, even if the original answer is quite short. Still not asking direct comprehension questions, but helps to get her past the starting blocks so to speak.
- And if he likes to draw...I had my dd doing a drawing narration yesterday which was fantastic. In general, I've been having her first tell me what her favorite thing in her geography book chapter was, and then draw a picture after. Yesterday, she was having a terrible time trying to tell me and I asked her if she wanted to draw first and then tell me afterwards. She did and it was probably one of her best, most detailed narrations yet. Better yet, today she asked when we were going to read that book again, mostly because she wanted to do another drawing-narration. We'll definetely try this method again soon!