I know how you feel. I am like that too in that I definitely explain myself and articulate my feelings a lot better in writing then I am able to do in person. My problem is that I overthink what I want to say WAY too much. If I try to do that live, face to face, I often feel like I don't do justice to what I want to say for one reason or another. Maybe I word it in a way that could be misunderstood, or maybe I stutter and stumble on my words and leave feeling like an idiot.
Even so, there are some conversations that just should happen in person, face to face. (Or, at the very least, over the phone). Obviously how you handle it is going to have to be your decision, but my personal opinion is that this is definitely one situation that should be a face to face conversation. Since you think you do better in writing, a good idea would be to sort of prepare yourself and think about everything you want to say and how you want to say it. Heck, it may even be a good idea to open up a Word document or an e-mail that you aren't going to send and type everything in a letter. Type up a message as though you WERE going to send it. Nitpick it all you want, edit it until you get the draft that makes you the most happy.
This will help you to figure out all the main points you want to say, how you want to say everything, what things you may have said that after some thought you may want to either not say or say differently because you think they may not come off well, etc. I wouldn't recommend you bring this letter with you and read it, it is really just to get all your ideas down on the page so that you have it all thought out before the conversation.
Being somebody who doesn't articulate myself as well due to shyness and uncertainty when in person, I will often have the important and/or difficult conversation face to face, but then follow up a day or so later with an e-mail/text conversation/whatever to clarify any points where I may have felt like perhaps I still didn't articulate it well, or may have given some sort of wrong impression.
Again... I'm weird.... I know it. LOL!
As far as specifically what to say, I wish I could help you but that is really something you have to decide for yourself. You need to know what works best for you, and also gage what you think would work for him.
The basic idea, though, is you want to make it clear to him that you don't mean to imply he has done anything wrong. Heck, it may even help to be somewhat apologetic. Almost as though to say something like "I'm sorry. I know this is goofy because you've never given me any reason not to trust you. It isn't that I don't trust you, but I just feel uncomfortable with my boyfriend being as close to another woman as you are with her. I don't mind you two being friends, but I'd feel comfortable with it if...." and the conversation continues from there.
Again, it is certainly possible he is just one of those guys who plays with people's hearts like that, and he will eventually hurt you. But, you've seen no evidence to suspect that yet. It is just as possible that he actually is a decent guy, and just happens to be nice enough to people that some people mistake it for something more than it is. Sad world we live in where nice people are so rare that people mistake somebody just being a good person for meaning they are crushing on them.
So, really, the idea here is just to let him know how you feel, let him know what would make you feel better, and gage his reaction to decide if he seems sincere or not. In time, you will find out if he is sincere or not anyway. If, worst case scenario, he is a "player" you will eventually find out anyway. But, if he's a good guy, and you can live with his friendship with other women so long as it is within reason, then why not him the chance?
Good luck to you, friend. Let us know how it goes. I hope it works out well.