In my first year, most of my labs were associated with coursework. We also had a lab series for one module that was partially marked on attendance. Missing labs was usually not allowed without propper extenuating circumstances (going on holiday would not be accepted). At the majority of universities, I believe the majority of labs are compulsory attendance (which is obviously very different to lectures).
Your DS is probably also aware that he may have coursework to do at this point in the term which has not yet been set by lecturers. Any group work, especially, will cause issues if he is not in this country. He may also be set something like a presentation to present during that week (I admit this would be bad luck but it is not impossible). As he is aware of this, he will probably feel committing to a week off is imposible.
It is not like school where there are lots of second chances given for late coursework.
Also, instead of reading week in the first year, I had field weeks, which meant I was expected to attend various visits to different locations, and take part in field work. This was also compulsory and the data collected formed a large part of the coursework for one of my modules.
Tell him he has to decide for the 24th. Tell him it will cost you money to cancel, and you really want him to speak to his tutor. They will not be cross with him for asking a question. However, it is unlikely a tutor would discuss his accademic situation with you unless he agrees to this.
However, I do think you have to accept that the answer may be that it's not possible for him to take a whole week off, and commit to this several months in advance, when he doesn't know exactly what work/deadlines he will have at that time.