I posted the first answer last night, but it did not appear. :(
I believe the time frame is unreasonable, and detention might be a abit harsh. That being said, as a teacher I do agree that he should do the work. I do not know the protocol in your son's school, but he/you should have obtained the work he would miss while having an extended break. It is difficult to always give advance work b/c there are a lot of factors that go into the planning and delivering of lessons.
How much school did he miss? If the class was able to start and finish a book of some length, it seems as if he might have missed quite a few days.
In the states, it is technically illegal to take students out for family travel. Though it is done all the time. I never penalize the students b/c children have no controll over the decisions made by their parents regarding their attendance in school.
If you know the title maybe I can help????
I would write the teacher a note and ask her to call you ASAP so you can discuss the matter. When you do speak, explain that you support the idea of his being held responsible Blah, Blah, blah, but the you would like a little more time. Ask her how you can improve home/school communication, etc. (We love that crap!!) If this does not work, go to her superior. He should not be held to a punishment b/c his parents took him out of school for holiday. He had no control over it.