You have agency over your own life. You can choose to not go on holiday with him, he can't make you go. You don't need STBXH's permission or agreement for how to live your life going forward.
He can book whatever holiday he wants. If it ends up being money down the drain because nobody goes on it or because he books a place for you and you don't go, that's not your problem.
You also can book whatever holiday you like. I suggest somewhere in UK if you don't want him taking DC abroad, else you'll find yourself facing similar issues he does.
I don't know all the ins and outs of child protection laws. Does DC have a passport? If not, get one ASAP and keep it away from STBXH's access. Refuse to let DC be taken out of the country on holiday. He'll have to take you to court to be allowed to do so. You can argue that it's not in the best interests of DC, who doesn't yet know you're divorcing, so would be upset at your absence especially if you're primary caregiver.
Can also say you've got concerns about parental alienation if DC and STBXH spend a week alone on holiday. Unless you think he won't be telling DC it's mummy's fault they're upset because mummy didn't want to come on this nice holiday with them. In which case it's presumably safe to let them go on holiday together, unless you've some other concern about his parenting.
Personally I'd tell DC about the divorce sooner rather than later. I suspect STBXH is either expecting you to back down on divorcing so is refusing to acknowledge it properly. Or is wanting to leave telling DC until he no longer lives full time with DC, to avoid having to deal with DC's upset over it. Leaving STBXH free to launch straight into playing Disney Dad EOW and leaving you to deal with the fallout of the divorce and look like the bad guy for separating DC from their dad.
Do what is in DC best interests regardless of whether STBXH agrees or not. Co-parenting doesn't mean getting each others agreement on everything. When you live separately neither of you will have any control over how the other one parents DC. If you can be in agreement, that's great. But if, as I suspect, he's a controlling arse who always wants his own way -realistically, being in agreement isn't always going to happen.