Ok. So I mean this is the most kind, respectful tone, so please read it that way if you can. And if I'm off base, just ignore completely.
In a sane world, a 9 year old would not even imagine that he has the choice or power to decide if he is going on a family trip. It would be a given, and he might raise a ruckus, campaign with tears, negotiations and drama, but it would be with the assumption that he was negotiating with the people in power of making the decisions. Somehow your kiddo has come to the conclusion he is in control and can make this kind of ultimatum, and for a child with anxiety, this is scary. He is very aware that he shouldn't be in control and isn't experienced enough to be in charge, so it actually makes the anxiety worse.
I've had two of my kids have anxiety to various extents - the youngest much worse than the older. Here are some of the things that helped:
1. We established firm rules with immediate consequences so they knew we were in charge.
2. We didn't ask when we didn't want the kids to think they had a choice, we used declarative sentences - we are going, you will be, to more the plan for all of us is...
3. We gave them acceptable areas of negotiation - several options that were all acceptable to us - we are going to our friends' house for dinner, and we're aware you're going to be bored since they don't have kids, so you can take one video game player, three games to play on it, two books, and one other item that you can run by us to make sure it is ok. That way you'll have something to do. They never got the option of negotiating whether they had to go, but we gave them ways that they could feel in control within the boundaries we set.
4. When one of them panicked or their anxiety escalated, we talked through worse case scenarios, what their plan would be if the worst happened, and I used humor a lot to break the tension. Once, right before a major competition, one of my kids completely freaked out about competing. We talked about what would happen that could be the worst - them being laughed at, breaking a bone, etc., and then about what could happen if they didn't compete - their team mates being angry, their embarrassment. And then I threw it the idea that they could still break a bone tripping on something in the parking lot. Unlikely, and it broke the tension. Somehow understanding that there were risks either way, they were able to conquer their fears.
Your kiddo needs you to be strong, in control, but still accessible and understanding. That is a huge order, and I didn't always succeed. But when I did, their anxiety was much less because they knew that someone who cared about them was in control and helping them cope.
