I have an ex-H like this. Constantly changes plans, lets DC down, promises holidays and then doesn't deliver.
Sadly, I have not found any solution that works in the last 15 years - in the sense of forcing him to make good on his promises. I took advice on here and tried every different tactic and couldn't get any to work.
So, I focused on my relationship with the DC and mitigating his damage. In your situation, I would be clear with the DC that you don't know if they are going skiing or not, as Daddy has yet to let you know. This is an honest statement from you that lets them know that there is uncertainty. You could also tell them, that you will get their ski gear out of the cupboards, just in case, but they may not need it - again clear message from you that you are not sure what is happening.
I also just carried on with my own plans, if there was any uncertainty and so would book to do stuff with the DC, if he was being vague and then he'd have to work around my plans, not me around his. I didn't speak to him on the phone, so I had a trail of emails or texts as back up. So, an example would be:
HIM: Margo, not sure I can take the DC away this week now, will probably have them for just a few days.
ME: Which days?
HIM: Don't know yet.
ME: OK, I've given the aupair the week off now, as you said you were having them. They'll be disappointed if they aren't going skiing with you, so I've booked them to go dry skiing with me on Thursday and a theatre trip on Friday. The rest of the week is all yours.
HIM: massive pissed off ranting, which I completely ignored.