I don't think Jack let Ennis go. Actually, I don't think anything really changed between them, but I could be wrong. In the short story, and I'm not saying the movie has to parallel the story in every sense, but Annie P. says, and I paraphrase,
They torqued things to where they had been, for what was said was nothing new. Nothing ended, nothing begun, nothing resolved.
I think if anything, Jack gave up the will to live. I truly believe that Jack didn't care whether he lived or died when he realized that Ennis would never build a life with him.