Well, first of all, he says, "I know where Brokeback Mountain is." I can expound on this if you like.
But later on in his talk, he tells about Jack's hopes and dreams that he and Ennis would move up to Lightning Flat and have a cow/calf operation there and a sweet life. He speaks with constrained intensity. Behind his words I sense an accusation: Ennis! Why couldn't you embrace your true nature and be with my son and be his partner. Also I sense an angry sadness that his son's hopes and dreams were unrealized. As a parent, I would have been angry at Ennis too, realizing that Ennis could have protected Jack from the hatred of the world. If Ennis had stepped up to the plate, Jack would have been happy and wouldn't have indulged in risky behavior.
Old Man Twist does say that Jack's ideas were "half-baked". I don't really see this as a condemnation but rather an acceptance that Jack had flaws and needed the complementary skills of Ennis, who could have planned everything out and executed it well.
I'm also thinking of a passage in the story where Ennis, meeting the elder Twists, thinks that he doesn't see much resemblance to Jack. This leads me to think that Twist pere never sired Jack. That doesn't mean he's straight or gay per se but it's another puzzle piece. I don't think any of these are new ideas, they've been bounced back and forth before.
