Maybe it was a "knee jerk" reaction by Jack, still angry and pissed at Ennis for changing the next fishing trip from August to Novemeber, he probably talked himself into thinking he could have something special with Randall, and by telling his father he was gonna bring him up to the ranch,was his way of making it more of a reality, and not just something going through his mind.
I think the suggestion to his father, was merely words Jack used to console himself, and not necessarily something that he had discussed with Randall beforehand.
What has always amazed me, is that Jack actually had conversations with his father about either bringing Ennis or Randall up to the ranch, and what amazes me even more is that his father obviously accepted that Jack had these idea to move to the ranch, and live with one of these men.
When OMT was telling Ennis about Jacks "ideas" about bringing him up to the ranch, and then "some rancher" up to the ranch, he didnt seem at all perturbed about the fact that Jack wanted to bring a MAN up there with him, he was more synical about the fact that they were just more of Jack's "crazy ideas", that never seemed to happen. The way he said it, was like he would have been happier with Jack if the ideas had eventuated.
I often wonder whether the mention of the "other rancher" to Ennis was meant to be cruel, or whether OMT really did not have any idea that those words would be so painful to Ennis.
When he blurted out those words at the table, I felt so sad for Ennis....it was like OMT was indicating to him, that HE was just another one of Jack's friends, another one of Jack's ideas, nothing special.......and I wondered whether Ennis maybe thought that was the case too, until he found the shirts, and that certainly showed him, that HE was someone more special than any of the others.
I have always thought, that was the reason Mrs Twist asked Ennis if he wanted to see Jack's room, because I think she had probably seen the shirts and wanted Ennis to find them. The nod she gave Ennis when he bought them down, indicates that she was pleased that he had found them, and knew that he would realize the significance of them.