Sorry to chime in, but I’ve done copy writing on a per document basis. That makes more sense to pay him by that. For example, we would receive a 2 page document that need adjusting. So the comp was $10 per page. It’s super easy to sell and doesn’t leave both parties hanging because there was time frame for the milestone to be completed when she received the initial plan. So handing over a page could be done in the beginning, middle or whenever needed. As long as, the phase 1 package was completed by “X” date. Never needed to deal with the company, just the project manager. Edits can be done by you. This allows for solid cost estimates, and the budget was never in question. I’d go per page.
Then you can work with him on the expected time. Give it early enough so he doesn’t rush. Explain the milestone for you, which can be 1 week earlier than the client’s. If he has a time management weakness, find someone else with the same parameters.
Hope this helps.