I'm going to insist that my teenage son joins a prepaid holiday he originally was very happy to make
Is there a reason for insisting on this?
I like my son to try doing things on his own and learn from the mistakes he makes.
It's understandable to teach your children to keep true to their promises. However there might be a reason her no longer want to attend. Try talking to him and finding out why he doesn't want to go.
I guess you are right.
Trust me, it's the right thing to do. He may be your child, but you shouldn't force anything on him.