steelchic, you need to stop trying to guess what he is doing with his time - ie. whether or not him and his GF have broken up. He is no longer your partner and your relationship is over - it's so hard, I know, but you are not allowing yourself to move forward here. It's actually not good for your kids in the long run, What your children are seeing (and what they will see if this continues) is that you are prepared to wait and hope that he will come back to you - while he does what he likes. That is not a good model for them - they need to have clarity. He is right - it's confusing for them and it's better they get to accept what is happening.
He is with someone else - you can't keep on having family holidays with him. That's not a sustainable way to live - and your holiday would be nothing like the holidays you had when you were together. You need, in your own heart and mind, to see this is over for now.
He obviously doesn't want to come - so why would you want him there? I really feel for you, but it's not fun to have someone with you, who would rather be elsewhere with is girlfriend. And - it's not about putting the kids first - you aren't together, and in the future you will have to plan holidays without him. Could friends fly out for a day or two?
Imagine being on holiday with him, with him sneeking off to text/ call - if he no longer wants to eb with you, how can you spend two weeks together, having dinner/ him sleeping in another room - it will be torture.....
see this as a chance to move on and accept its over....he is trying to comfort you by spending time with you and getting on - that doesnt mean he wants to be with you. because if he did, he would be and he isn't.
you have a wonderful life ahead without him, remember how unhappy the relationship was and try to move forward. If anything, I would even say cancel the holiday if you really think you wotn enjoy it.