Aaah, the joys and limitations of board postings, especially as a first-timer! So difficult to give context without rambling on. Thanks for all your comments.
I do always discuss holidays with DS before booking, father never does. DS knew nothing about this holiday until phone call today. I have always shared school holidays; every leave weekend and half term is split equally, an arrangement my son chose so that there wouldn't be long gaps between seeing either parent. Most of the longer holidays are shared that way too, particularly since his dad retired and is more available (and also therefore has more flexibility about when he books holidays away, whereas I have to negotiate with colleagues for time off). I have never limited his access. Yes, I would cope with the longer gap if son wanted it; I have asked him in this instance and he says he feels it is too long, but he's such an easy going guy and I think he takes the line of least resistance.
Still think it is reasonable to discuss and agree arrangements in advance, and yes of course involving DS as I always do. The context to all this is a very long history of similar incidents: DS's dad making plans without any consultation, failing to honour agreements made (and denying there were any such agreements), changing plans at the last minute, fabricating conversations in which I have agreed to his plans (which is why I asked for everything to be in writing in the first place - less room for those kind of misunderstandings) to the point where I think I could be in the remake of 'Gaslight'!
BTW, it was DS's choice absolutely to go to boarding school - our local schools are uniformly awful. He has never regretted his choice and is really happy there. He knows beyond any doubt that if he ever became unhappy, he could return in an instant. Believe me, as the mother of an only child, it is sometimes unbearable spending so little time together and I miss him so much, but seeing him thrive and grow there is worth it.
So much more to all this but I have rambled enough. I suspect there is no answer to this other than me finding a way to manage my frustration at having to deal with someone who just doesn't do co-operation and delights in pushing my buttons - my punishment for leaving in the first place I suspect. Hey ho.