Going by what he says to you and by his resistance to write such letters he’s not comfortable doing so. Likely because it means he has to face his death.

He might not be comfortable with it.

Often people, who know they are dying, write letters and make recordings for loved ones. Yet it’s typically initiated by them, not their loved ones.

I do understand you wanting such letters or recordings so I get where you are coming from, but we have to remember that yes they are our therapists, but they are also human beings with their own feelings, fears and lives.

You might have to accept that he’s not ready to write such letters or make recordings, if he’ll ever be ready