IMO, speaking with ER is not to get her to believe him. She has questions and needs/wants answers. I've thought about what could be gained from them both speaking with each other. I'm going back to day one, before all the finger pointing, accusations, everything. Day one, your child is missing.
Questions I, as a mother would ask
What did you do from the second you picked him up until the second you left for your errands.
What did he eat at McDonald's
What did you buy at Walmart
Did he brush his teeth
Was he up late watching tv
My point is, I as a mother, would want to know each and every second of my child's activity. You never know if something that dad saw as totally unimportant would make a light go on in mom's head and potentially lead them to their son. I would want to know and let it stick in my memory like my favorite book, not forgetting a single moment or word my child said before he disappeared. He, as the father, as the ex husband, as a human being, should be willing to give that to the mother of his missing child. Not to convince her of his innocense, not to do anything other than give a grieving mother something. To have compassion.