Jack being Jack, I think he'd have said something or hinted, if not while they were up on the mountain, then after they came down and before they separated in Signal.
I think it came together in his mind once he realised that the Newsomes were never going to accept him, and that he was just a gofer in the short term and the guy who'd provided the Newsome heir, but unimportant in the eyes of LD, his wife and Lureen. Once again, as he'd been in his own family, Jack was the outsider who couldn't feel a sense of being loved and wanted, so he thought back to the time when he did, and realised that it was up on Brokeback with Ennis.
And since he was a natural optimist he decided to go back and see if there was any hope of rekindling what they'd had - LD had evidently already made it clear that he would pay Jack off if he'd go away, so money wouldn't be much of a problem. I think finding Ennis with two children must have been a bit of a shock, spcially after the passionate kiss that they'd just shared.