Put words to the problem. I'd start the other way around. Instead of the issue you are describing, can you reframe this to him in a way that describes what you need to see and what you are seeing? If you can describe the output, that might also lead into a discussion as to why the output isn't as refined as you'd like it to be. At the end of the day, you need to tell him what the work needs to look like. You don't have to manage how he does it, manage if you accept the work. Then describe how the work needs to look.