Like how to make changes to the schedule. I would want the Ex to give up time PRIOR to getting their change. You want DS 3 days early next week? I will pick kids up 3 days early this week. That's today? Guess What? Hmmm.
True emergencies you just work out for the sake of the child... if you end up a little short, that is just the way life is... it will all eventually even out.
And you can just say NO when it is your time. Your AGREED court order
is going to say something specific. You have a right to be a hard nose about your time. Sometimes you get flexible and you agree. Sometimes you take a hard line and you don't. No judge is going to ream you for using your time ... especially when the father has PLENTY of time to make his plans with his family on his own time. However, if you always say no, then you could also have a problem.
It also sounds like you are having more of a problem with there being no reciprocity and short notice. Train the ex to give you more notice. If he warns you way ahead, make it happen if you can. He gives you 2 days notice, say no unless it is a once in a lifetime opportunity for son.
And do not stand for the excuse that he talked to son first and son is expecting it. That is like saying to the lottery commission that I told my child I was gonna win and now you have to pay up. It is ridiculous to think they would.
You are an adult ... the parent. Sometimes you have to be the bad guy in real life. Well, this is real life. Your child needs to see real life all the time from you both. Real life is a series of negotitations. You let him see that sometimes you get what you want in the negotiation and sometimes you have to sacrifice this one for the next one. He will learn valuable life skills from you remaining civil and still saying NO... or from dialoguing a problem and finding a good reason to say YES.
Instead of asking your child whether he wants to go (or cares about going), ask him what are some good reasons for going. Then ask him what are some good reasons for NOT going. His answers will tell you what he really thinks.
Remember, you are not raising your son to be a child, you are raising your son to be a man. Keep that foremost in your mind when you have these conflicts with your EX and it will help guide you in how you interpret your son's "best interest." Look at not only how this choice affects him this weekend, but how it may affect him when he is grown.