dare I say this....
You've allowed a behaviour to continue without telling him to stop. (according to what you posted) You had not told him that the behaviour bothers you. As you've said you played along with it, so he wouldnt really had any idea that he should stop.
You said yourself you didnt draw any boundries, I think its only fair to this guy for you to tell him that this bothers you and see what he has to say about it. If you DID explain that in your letter only, IMO that wasnt exactly fair...you didnt give him a chance to honor the fact that you didnt want the behaviour anymore....it sounds like you told him....you did xyz (when you'd played along with it the whole time) and I dont like it so we arent gonna be friends anymore.
Hell, if I had been the guy, I'd have picked up the phone and called you to....cause I'd have been confused as heck about the situation.
Thats just my take, given the information you gave us
