the school will probably allow this time off if he has otherwise good attandence, some head's actually agree that holidays are actually educational,
i agree witht he last guys suggestion, offer to pay the difference in their costs, because this is usually a big factor in why people book hols during school time, i must admit i do it myself! Although the last 2 weeks of school they dont do much work, but, i see your point missing them will be like not saying goodbye to friends and teachers.