There's a couple ways you could do it.
1) You could schedule a 'performance review' or 'touch base' or whatever you want to call it meeting and go over everything (positive and negative... don't pile on with the negative) and praise him for what he does well while clearly defining your future expectations from him and making sure he's on the same page as you.
2) Every time you have to do one of his duties for him or correct something he messed up, you can email him and let him know so that he's aware. You want to be careful with that one if the guy has stress issues though... might push him over the edge and backfire. Me, as an employee, I prefer this one and I've told our senior management if I fuck up... even if it's a small procedural thing and not a major issue... they better let me know so I can fix it. Some of the people I work with get really offended if senior management calls them out on the little stuff though. That's a tricky one, really should only be used when you are sure you have a read on the person.
3) If you have time to do it, you could always take him under your wing a bit and mentor him on the aspects of the business that he doesn't seem to be able to pick up on his own. Schedule some time to meet and go over best practices. Maybe you show him some 'demo' letters and tips and tricks for how to perform better at those aspects of the job.
Just some ideas... when I had employees that weren't getting the job done my first inclination was to meet under the umbrella of a 'general performance review' and subtly ask them why. I wouldn't shut the door and throw the fuck-up in their face. It was more of a, "here's the good stuff, here's the bad stuff, here's what I expect from you going forward" and then based on their response I would figure out how I would proceed.
I'd say a good 50% of the time they weren't even aware that there was an issue, and it would get solved simply by me going over the thing they fucked up and them saying, "oh, I was taught not to do it that way... I'll do it your way going forward".