This is a really great thread.
I tend to agree with those who say that Ennis would never have completely opened up to Mom; he would leave most things unstated. But they could still have shared plenty of memories. Mom especially could open up and tell most everything she remembered...stories of Jack's childhood, how Jack acted whenver he would visit them after the "fishing trips" etc.
Ennis, on the other hand, could tell about the harmonica (maybe ask how he got started with that), about the sheep, about the elk, about Jack's singing.
And (not opening that whole can of beans, I mean worms) if Mom is a Pentacostal Christian, I really don't see them having a frank discussion of Jack's sexuality. Even if Ennis wanted to, which he didn't. They both labored under their own limitations they had partially created for themselves.
And, I mean really, I have no life experience upon which to base this question, but maybe somebody does.... you don't really discuss the intimate and sexual side of your relationship with your lover with the lover's mother do you? I don't see that as commonplace even in today's society. (And especially if the lover is dead. No way.)