If he told you that "you should know better because you were an elder and know it's the truth", I would tell him in the letter that it's precisely BECAUSE you were an elder that you know it's NOT. Tell him that you saw the pettiness of men fighting for position and power, that it was obvious holy spirit played no part in appointments, and that you found the child abuse coverups completely unacceptable.
He can put that in his pipe and smoke it and hopefully it will ring a bell somewhere in his head, telling him to pay attention.
So sorry you and your wife are having to endure this. I still have one child "in" but at least I've only been called an apostate once, which I refuted [use of the term], and we get along even though we don't share beliefs. There is hope for you two, too!