AIBU to say No?

[Geraldine22]

DS Dad wants to go on holiday in term time, in the March following DS starting Grammar School. Holiday of a lifetime no less travelling round America for 2ish weeks. He's not paying his grandparents are, and they are fixed on the dates purely due to the money.
I really want him to go, but have asked they change the dates to incorporate some of the holidays in there to lessen the blow to his attendance and the amount of work he will have to catch up.
What is now happening is threats to call the school, Im being unreasonable, How dare I suggest this to him, and if he cant take him out of school then I am not allowed to either. I have told him I'm not planning on ever taking him out for 2 weeks, but it's not sinking in.

Even though I have asked re: date change, he is going to call him later to tell him all about it, and then I have to tell him he cant go.

I've had to phone the school this morning, that he isnt even at yet to explain.

AIBU?

He can't get out of meetings now by claiming distance. This whole lockdown has proven things can happen perfectly fine over Zoom. So no excuses there. He wants to go he needs to take the responsibility for it.

@Geraldine22

He cant take him without permission as I have a court order where I am the named resident parent. He needs permission, his birth certificate and passport, which I have.

He will go with or without him, he's done it before and he will no doubt do it again, and then rub his nose in it with photos.
I

How horrible.

'I'm so sorry DS. I really begged Dad and Grandparents to try and change the times so that you could go, but they couldn't. I spoke to the school and as I expected, they don't allow holidays in term time, and if I broke the rules and just didn't send you to school for that length of time, I would be fined a lot of money and could even go to prison - it's not as if it's just a couple of days. I'm really sad about it too. I wish that Daddy could have changed the times as I would have loved for you to be able to go'

I agree with @YoniAndGuy

"How horrible.

'I'm so sorry DS. I really begged Dad and Grandparents to try and change the times so that you could go, but they couldn't. I spoke to the school and as I expected, they don't allow holidays in term time, and if I broke the rules and just didn't send you to school for that length of time, I would be fined a lot of money and could even go to prison - it's not as if it's just a couple of days. I'm really sad about it too. I wish that Daddy could have changed the times as I would have loved for you to be able to go' "