About his decision, notify, in the first lines of his letter. If he wants to find out the reasons and motives that made you make this decision, he will read it next. In that case, if it will be enough only the information that you need to leave, he will not have to waste your time figuring out the reasons.
If you think that your motive for it is still important that we give them. Do not describe special cases, try to summarize those features in his character or behavior which make co-existence impossible. You have very lucidly explain, what your love has passed, and no chance of her return is gone, even if some of the flaws will be corrected
Even if you already know someone to replace him, it is not necessary to explain your gap to the advent of new men, it is not necessary to inflict additional blows to his ego. In addition, this argument does not put an end to the relationship, and allows us to hope that if the opponent is eliminated, you can be together again. You just provoke a showdown between his men and the fact that the new lover will want to fight for you in the presence of such an active fan.
Do not refer in his letter on the opinion of friends or parents is also only embitter him against innocent people wanting you to be happy. And do not write stupid phrase "let's stay friends", because in such a situation, it will be humiliating for the person rejected. Take responsibility for the decision solely on himself and put a solid point that does not allow to hope that you might change your mind.