“Practical” to say so? Meaning will he disobey and go anyway? If that’s your concern I think you have larger issues…
However, if you meant a) is it practical for him to go, that’s going to depend on your family situation, or b) is it reasonable to say he can’t go, absolutely unless tickets have been booked for months and you already said yes a while back (barring extenuating circumstances).
It would be a lot easier to answer if the question itself were clearer.