I'll be the dissident and say go visit him. Not because you have to, certainly not because you should feel guilty!, not to get involved. Just because it really is your absolutely last chance to see him and you never know what might come out of it. Not miracles, just a little bit of ease connected to your relationship that may make you feel happier in the years to come. This is assuming that HE actually wants to see you. It sounds like he does regret losing contact, has given up, but his more emotionally savvy ex can see it.

The way I read your post you're mad at him, not indifferent. With good reason! This seems to me your only chance to do something about it - for your own sake, not his.

Good luck with your decision! This situation will take energy no matter what you do, I think.