start by thinking of little things you can do to take back control.
Do you have any sort of formal, court agreed custody/contact agreement? FaceTiming multiple times per day would almost certainly be classified as stalking, harassment or coercion. Find some advice. Start a thread in Legal here asking just this question.
Does your son want to talk to his dad that much and is he happy when doing so? Do you have a designated spot for him to talk to his dad? Start by finding a neutral spot in the house where it’s a blank wall only as a background and there is no answering FaceTimes until he is sitting there. Make it boring for ds and he might start pushing back against it himself. Ds shouldn’t be free ranging the house while FaceTiming.
At the age of 8, what device does ds have access to that supports FaceTime anyway? At some point, can you say school have recommended that ds at his age is too young to have such open internet access. He will be available at this particular time each day for you to ring him. See above comment about stalking.
Someone on here can tell you the details of Court approved parent contact apps. Download that, send him the details and start communicating through that. He won’t like it but at least your end will be in a Court approved format.
Re your holiday, think about how to swerve whatever disruption he might plan. Can you and ds leave the night before and have a cheap night somewhere on the way? Or leave early, take a longer, scenic route and stop for breakfast on the way? Something to keep him off balance.
Where do you do handovers? At your house? Try and move it to somewhere neutral. “Yes, I know it’s been at home previously but this time we will be out early and ds needs his time with you. We will have his bag in the car”. Don’t ever say why you will be out or where. Ds might but that’s after the fact. Ex only needs to know that you will be at McDonalds at the appropriate time.
Make ds coming home as neutral, boring and as quick as possible. “Hello darling. Come in now” and shut the door. No conversation with Ex. That’s for the communication app.
You need a neutral, say nothing response to everything he says. “noted” is a useful word. “If you introduce my son to another man, I will go for full custody “. You say “noted” and nothing else.