As hard as it is....you may need just stop answering his calls. He knows what he needs to do and he just doesn’t want to do it. Every alcoholic says they want to get clean, they just don’t actually want to do the work it takes too get clean, which really just means they are not ready to get clean. You have told your son he can come home with the necessary boundaries in place. He doesn’t want to accept those boundaries so why is he still calling you? To play victim and get sympathy and manipulate you. It doesn’t help him and it certainly doesn’t help you. You talking to him doesn’t accomplish anything other than you knowing that he is still alive and drunk and it causes you great emotional stress of course. The reason his recovery efforts haven’t worked is because he is not really ready to quit despite him saying he wants to. He just wants a magic pill and everyone else to fix him and that isn’t going to work unfortunately. I would honestly go no contact. He knows what he needs to do to get help, no one else can help him get there. Of course you will worry about him but it sounds like you worry about him even more when you are in contact with him and it doesn’t change anything. If he doesn’t get on the plane, oh well. It doesn’t change anything for you whether he does or not. Chances are he will be drunk and they won’t let him on anyway.
What are you doing to help yourself with all this? Are you doing counseling? Alanon? You need to focus on you and not him because as you know it doesn’t help any. I know it is easier said than done to cut contact with him but it will be much better for your mental and physical health. He is an adult, what he does is his choice even if that choice is a very bad one. He has the choice to seek help and get sober. He just chooses not to do that. That is on him and no one else.
Take care of you and be kind to yourself. This is really hard to watch but at some point you have to accept his choices, crappy as they may be.