Most Helpful Guy
Actually the shorter the better. Long letters can be annoying to read, and it won't beat around the bush if you just keep it short and simple.
All you need to say is "hey, we need to talk and you're not going to like it". He'll respond with OK what's up and then you just say "look, I've had a nice time with you and all that butttt I feel like this just isn't working out, the long distance thing, I hope you can understand this and I hope we can remain friends (though it's nearly impossible to stay friends with an ex)". That's all you need to say. He may try and persuade you to stay but as long as you stand your ground, you'll be fine