Adam, I run into customers like this all the time. He had an idea in his head that you were going to use wiremold because one of his neighbors installed a receptical using wiremold in less than an hour. So when you spent several hours installing his receptacle the correct way as we know it. He was a little shocked and unsure if he was being taken advantage of. Once you explained to him that you performed his installation correctly as the code is written. It sounds like he came around to your understanding.
Don't expect him to admit he was wrong for questioning your work. Just accept the additional work he gave you as a sign of his new respect for you.
