My take is that he finally gave in (to the "whatever " it was between them). Remember the "look" Ennis threw at Jack when Ennis left the main camp and Jack asked, apologetically almost, "See you for supper?" after their *first night*? It was a look of disgust. Ennis was still not accepting at the moment. (In the book, it's a bit different, but essentially turns out the same way)
Then Jack went up to the mountain. A better bet would be Ennis didn't and didn't want to return to the main camp that night, so Jack went up there the next day and basically saying "let's talk" -- in their cowboy/herder/ranchhand way -- prop up on one elbow and side, then wait. :p
Of course, this could all happen on the same day.
Now that they "talked" and understood each other, so Ennis returned to the main camp. Jack couldn't just *pull* Ennis to the tent after the supper, so he went in first and whatever. Jack switched the role and was the camp tender anyway. Ennis could just head back to the sheep, but after some hesitation, he chose to continue the "courting". Of course he was really new at this, hence the "apologetic" and "vulnerable" look and behavior. (I'd say Ennis's being "apologetic" because the "mean" look he threw at Jack that morning)
BTW, I thought I heard (almost non-audible) Jack saying "shhh.. It's ok, it's all right" during the 2nd tent scene...