That's kind of it though, isn't it? If you're going to convince him there has to be a reasonable chance that the opposite will happen and that he'll convince you that he's too old to learn new tricks or whatever and he's going to have to do it his way because he isn't up to the challenge of doing a better job.

If your range of outcomes is [He'll do things my way, There'll be a scene and a strained relationship] then sometimes there'll be a scene and a strained relationship. If the range of outcomes is [we do things my way and he hates it, we do things his way and I hate it] then that's at least softer on the relationship. If you're lucky maybe you don't even care and you can just live with some parts of the work being bad.

One of those awkward things is that being good at negotiating means that other people are more likely to get what they want. It is actually a bit counter-intuitive.