I would personally do the old fashioned 5 paragraph essay type of deal.... Open with your love for him and how proud you are to be his partner, move into 3 (or however many you have, even if only one) important points you want to make, and then conclude with more love and affirmation. I think looking at it from his perspective is a great idea. Write it from your viewpoint, then read it over as if you're him. Does it feel like an attack? Does it make sense and make a valid point? More importantly, does it give him something to work on? If you only point out problems without proposing a solution, you could make him feel helpless without wanting to. He may not know how to do/be the things that will fulfill you in the relationship. Try to visualize what could change and then write those things down specifically.

He's lucky to have someone that cares enough to go through the trouble to do this for the relationship. I hope you have some good news for us in the future!!