Del and Barb,
Gotta love both of you! You both always explain your cases so well and persuasively! I'm not, however, persuaded in this case. Nor do I love Jack the less. It just hurts all the more, now I see him in this light.
I think it's possible that what we have here might be a failure to agree on what it means to grow up and what it means to be a man. It might also be the old and perhaps irreconcilable conflict: what one person sees as being true to one's self, another person sees as being selfish. Perhaps it comes down to what has been said often about this story. Ennis and Jack were in an impossible position. There were no winners, and in the end everybody got hurt--Alma and Lureen as well as Ennis and Jack.
Granted, Ennis was a lousy husband. For that matter, I've never really understood why Jack stuck it out with him for that long. Wouldn't it have been more "grown up" of Jack to quit Ennis long before that final confrontation?
I'll grant the point that Jack does seem more willing to try to fix things than to stand them, but look at his suggested fix to the food situation: killing and eating one of the sheep they were getting paid to guard. I have to take my stand with Ennis on that one. Let's also not forget the first part of Ennis's famous phrase, "If you can't fix it." That doesn't mean, "don't try." We don't know whose idea it was to try hunting (maybe it was Ennis's?), we only know that Jack was a bad marksman and Ennis finally resolved the situation by shooting the elk.
(OT but I've always felt Jake was badly directed--yes, I said badly directed!--in the scene where Jack tries and fails to shoot the coyote. He isn't even trying to aim that rifle properly.)
I think the evidence is slim to call Jack his son's primary care giver, but that might also hinge on how the term is defined, though without doubt he does show admirable concern for his son's well-being. (Let's also not forget Ennis's concern that his little girls are being exposed to those slopbucket bikers, he suggests treating them to ice cream, and he promises to take them to the church picnic.)
We don't know whose idea it was to go to that charity dinner dance, but I wouldn't be surprised if Lureen dragged Jack there, more out of concern for social position in the community than from any desire for a social life. To me he doesn't look too happy about being there.
And running off to a hustler in Mexico just because he can't get laid by Ennis is the act of a spoiled, selfish brat. (Yes, I'm being very harsh here.) Running up to Wyoming unannounced just because he got word that Ennis's divorce had been finalized is not a very adult course of action. I feel for Jack that his sexual needs are apparently stronger than Ennis's, but I'm not therefore excusing his behavior in running off to Juarez.
I hate--I really do hate!--to disagree so substantially with both of you, but I'm afraid I have to. In my world view, Ennis is much the more adult of the two.