First thing I notice is that I never see a letter received well that has insults in it. I do not think it will be productive if you accuse the other party of having "narcissistic tendencies," of being a "pathological liar," or accusing the other parent as you have, no matter how true it may be. I recommend following an entirely different format.
First, you non-confrontationally describe the situation in concrete terms without judgment. For example, give specific facts that are of concern. Here is an example: "Our son told me that you have been trying to get him to spend time with him outside of the time proscribe in the parenting plan. When he told me, he said he felt pressured."
Second, express to the other party how these facts or examples make you feel. Again, in a non-judgmental fashion. Just factually state your feelings. Example: "This makes me feel sad and frustrated."
Third, clearly state what you are asking the other party to do. Give specific examples or make specific requests. Again, non-judgmentally. Example, "I am asking that you not pressure our son into spending time with you outside of the time proscribed in our custody order. Instead, I ask that if you want to make any exception to the order, that you communicate with me about this in writing and you and I can discuss it."
Fourth, emphasize why compliance with your request would not only benefit you, but would also benefit him. Example: "I'm happy to make variations that are in our son's best interests, so if you and I communicate, some of your requests may be granted. Also, by not including our son in this communication, we can take the stress off of him, which I am sure will make both of us happier."
Does this make sense? It feels good to tell a liar that they're a liar, but it doesn't ever get you what you want.