Say I love you, and then tell him WHY you love him. Is so, make a list of those things and make that list a part of your letter. End the letter with a call to action. Ask him to call you, meet you somewhere or just hold you next time you see him. There is no magical formula, M. Just follow your heart and pay attention to your intuition.
Nothing is more powerful than honesty and sincerity.
Don't just say I love you on paper or in words. Let your love be evident in your actions. No matter what the issue, he will answer you honestly and empower you to make the best decisions.