Do you really think he was looking for a replacement for Ennis, or just someone who he could have a drink with which may lead to more.
At the time Jack approached Jimbo, it was during the four years that he did not have any contact with Ennis, so he did not know if there was ever going to be any future with Ennis at that time.
I think the scene shows us, that during that time, Jack was approaching other men, and maybe sometimes he got lucky, but this time he did not.
Maybe he just wanted to be "one of the boys", wanted someone to have a drink and a chat with. Thats why when eventually Laureen started flirting with him and paid him some attention, he grabbed it, even though it wasnt exactly the gender he was really looking for.
I do think with Randall, that that was a replacement, or more the opportunity to have someone nearby who he could "camp" out with or go "fishing" with, and maybe have a future with, as he had told his father, that they might move up there.
If times were different and opportunities more frequent, I think Jack would have been very promiscuous. The "high altitude fucks, once or twice a year" with Ennis were not enough for Jack. He needed more sex than that, and if he could have got it he would have. And when he didn't get it for free, he went to Mexico.
But I doubt if he ever would have had the feelings and love for anyone else other than Ennis.