I'm assuming you are communicating online only, and that you're not paying him. Is this right?
I would just send him an email and let him know you've decided to go in a different editorial direction. You don't have to make excuses or anything of the sort. Just thank him for his time so far.
I am an editor, and I know well that sometimes personalities, styles or skill levels don't mesh. He'll have to learn this at some point, and it's OK to be the one to teach him.