This must have been Ennis's first real taste of love. He didn't recognize it for what it was while on the mountain, but the minute Jack drove away and he was left alone, walking down the road, the enormity of his loss hit him causing him to shake, get sick, vomit, get angry, beat his fist on the wall, etc. When did he fall in love? Probably it had been creeping up on him over the summer, then caught him good after the FNIT. But it didn't affect him until he saw Jack drive away. He STILL didn't see it for what it was then. It took another four years before he finally figured out that what he felt for Jack was love. Perhaps he realized he was missing something in his feelings for Alma, and that something was what he felt for Jack. That had to be love.
I agree that Jack fell in love much earlier. You can see it in his looks toward Ennis from early on in the film. He was watching Ennis intently from the second he got out of his pickup and looked at Ennis standing by the trailer door. Maybe Jack was always open to finding a new man. Maybe his past had included many encounters, and maybe a few relationships with men, so he was always open to a new-comer. Whatever the case, he was eyeing Ennis from point one. Falling in love, however, may have happened at any time after that. I think he loved him while on the mountain. That's why he was so hurt when Ennis punched him on their last day there. He was already in love and couldn't imagine why Ennis was so angry that he hit him so hard. Just MHO.