The big deal that I see is this:
He gave up 2 days at Thanksgiving in exchange for 2 additional days at Christmas. Now, he's not getting his 2 days at Christmas.
If you made plans to get your child the day after Christmas, had everything planned, etc. and then your ex decided, "Whoopsie!! I lied! You're NOT getting those days!", wouldn't it be a big deal to you?
She agreed to this, then took it back. That would be like my ex saying, "Yeah, sure. I'll give you the 2 days after Christmas so you can have that "Christmas" time, if you'll let me have 2 extra for Thanksgiving" and then, after getting those 2 days he wanted telling me, "Well, ya know. I changed my mind. You can't have that Christmas time with the kids. You're gonna have to wait until the 28th. Oh, you were going to your dad's on the 26th for your family Christmas? Too bad. I'm keeping them those 2 days. Forget what you had planned."
You're damn straight I'd be upset and it would be a big deal to me. Whether I was a man or woman, NCP or CP.
We teach our children not to lie and to keep their word when they make a promise to someone. Too bad some of us adults can't practice what we preach.