Okay, this may be off track here, but what about letting him write about anything he wants. Basically, you give the guideline that ‘writing time is for the next 20 minutes’ (or however long/short), and that he is to spend his time brainstorming by writing lists of things (give a starting topic, but tell him to write and shift as desired, ex: beginning topic for brainstorm is natural disasters, but he ends up thinking about animals after listing a few disasters and the list shifts to be animals he’s seen at the zoo or wants to own, etc), telling about something he loves, or a recent book he read or movie he watched, describing a place he has been, making a wishlist of places to go, or retelling a favorite fairy tale, etc. When he is done, put his writing away. Congratulate him on working on writing. The next day you want to work on writing, hand back the notebook, have him browse past work, and then turn to a fresh page and write again, about anything. Repeat, repeat, repeat. In a few weeks, have him flip through and choose something he has written that he wants to ‘finish/polish’. This may be a description of a place he has been, which he can re-read, add details to, and then rewrite and illustrate. Ta da. Go a few weeks free writing again in the same notebook. Then have him flip through the notebook and pick something to polish up again. Maybe this time he wants to send a letter to a relative with a few favorite jokes. You get the idea.
Is this obviously helping with moving to written narrations? Maybe, maybe not. BUT if he can learn to use and enjoy writing, it will eventually become more natural to him to write things down, and THEN written narrations become easy.