mulranno you should pick your battles.. and still, considering all you have said (which is but a mere scratch on the surface of "how things are") and without knowing the personalities... I would still say, you need to stand firm on this one.. because it is simply not on to dump this on you at such short notice, without time for discussion, without arrangements being made, the fact that you will have to put upon family at short notice to take him, (no matter how much they love him etc they must have their own things to do), he must learn to negotiate properly and sensibly and that does not include leaving it until the last minute.
Learning to be an adult sometimes means doing things you don't want to do, not saying "I'm not doing it" and then getting the authority figures to rush around making it possible for him to not do. He will have teachers, tutors, university lecturers, friends, colleagues, managers etc all coming up in his life and he needs to know you don't get things by clicking your fingers like that.
I think the lesson you should be teaching him is that, if he has strong opinion on something, then he needs to a) anticipate b) negotiate c) compromise. For next time. This will be far more valuable to him in the long run.
As an aside, at 15 this sounds like the last family holiday you have together. What I would try to do is look at if you can make it any more fun when you get there so it's a really good one!