HI, ep, Sunny asked me to stop by and answer your thread. Your question is urgent, so what are your current custody arrangements? What would be the impact of you just telling him you were going to do what you want with the travel arrangements? How much has your parenting arrangement been formalized?
In my case, Mr. Fantastic rarely asks for extra time with the kids. I have them full time, minus every other weekend, so my plans are really never disrupted by his needs. If your H is more controlling I'm not sure how helpful I can be, but in general, I will make a couple of observations:
1. You've asked him three times. Any more than that and not only will he (unreasonably) think you're a nag, but you will also look very weak in his eyes. I've learned from parenting that you make your stand, make the consequences clear, and then you execute as promised. Not threatened. Promised. Threats are what you've made when you don't follow through.
2. I felt better about myself when I acted as a stronger person. Not in the sense of lashing out, but in the sense of knowing your own boundaries and honoring them. It took some trial and error to figure out how to do that, but it really helps.
One tip I've picked from elsewhere is that you use his desire to look good as a tool to motivate him. As much as you might feel icky doing it, flattering him into letting you swap will probably be effective. Something along the lines of what Sunny said, or "My family would so appreciate it if you give us the extra day to enjoy S4, and he'll be so much better behaved if the trip is less demanding." The implication here is that his child and his generosity makes him look good, even though he's acting like a... I'm sure you can fill in the blank here.
Worst case scenario, he doesn't give and you make the trip in one day. If that happens, congratulate yourself for being stronger and more resourceful than you knew. The summer Mr. Fantastic left me, I took my three kids on three different 10-14 hour (each way) road trips. Not only am I proud of myself for being Awesome, but we have some fun memories together that are a big part of how close we are as a four-person family. You've got this!!!
Me42, H40 D12, S8, S7 A revealed: 7/13 Sep 4/14; Agreed to D 1/15