I feel for you. I had a dear friend drop out on me like that. We were very close, talked and got together all the time, and she was a bridesmaid in my wedding. Her career took off and I knew she was busy, so I understood that and made an effort to stay in touch. She was always friendly and interested when we spoke even though she rarely returned my calls.
I realized that we were no longer friends when she never called after I had my daughter, and then six months later when my mother died. Even though I called to let her know about my mom (who she knew well), she never returned the call, was a no-show at the funeral, no card, nothing.
This hurt even more since I had supported her when her dad died.
I have only contacted her once since, to let her know that a mutual friend's father died. No response of any sort.
Sorry, don't mean to take over your thread with my issue...
but I think it is just cruel when someone breaks off contact like that. You deserve some explanation from this friend.
I think what you've done so far makes sense. If you write the letter, I hope your friend will see how sad this has made you and at least tell you what's wrong. However, after that you don't want to allow this to take over. Continuing to contact him with no response will only hurt you. If you see mutual friends, ask about him and let them know that you miss him. Hopefully, in time he will get past this. Good luck.