When you have someone like that, you've just got to be brutally honest with them as politely as you can. Tell them you're the expert. People judge you by the quality of your work product. Looks, aesthetics, etc. count. And that his vision of what it needs to look like is significantly outside the range that most would judge as normal and doing his way would harm your reputation. And you can't do that unless he pays you enough money to retire on -- for life. Thank him for thinking of you, tell him you're sorry it didn't work out, and give him his money back. (And don't let the door hit you in the ass on the way out!)