The thing is, Cloudkitten and others, this is precisely why parenting teens this age is such a challenge. The bottom line is that if mulranno's DS won't get in the car, mulranno can't make him. Seriously, even if it were physically possible - and many 15 yo boys are bigger than their parents, so it isn't - using physical force to get him into a car unwillingly would be totally unacceptable, and teaching dangerous lessons about getting things done using violence.
It doesn't matter how outraged a parent is, how they threaten or cajole, how wrong they think/know it is to 'give in to blackmail'... It has nothing to do with 'just saying no' or 'taking the strops on the chin'... It has nothing to do with sanctions and consistency - you can crack down hard and be absolutely firm, clear and consistent... If a teenager refuses to do something, despite all that, you can't make them.
Those people saying "He must come" or "He'd be coming if he was my son" are lucky enough not to have 15 yos who want something so much, or are wilful enough, that they are prepared to defy their parents even though they know they will be in deep trouble.
Those of us who have had teens like this know the op has to find another way. Even if she 'wins' this one, she will have to find another way pretty soon, because his DS will react like this again.
Even if/when you enforce sanctions, that still may not get them to do something they don't want to do. And some of them are prepared to put up with a lot of sanctions, to assert their autonomy!
That's why it's crucial to 'pick your battles'. You pick the ones you know you can win. You still may not be able to 'make' them do what you want - at this age they have to make themselves. If you can't offer them good enough reasons for doing something they don't want to do, then you're stuffed if your approach is just threats and sanctions, because they simply do not work.
In this particular case, the OP's best bet is to give him good reasons for wanting to make himself get into that car. No doubt she's trying that already. If she thinks there are no reasons 'good enough' for him, then she really, really is better off allowing him to stay, however reluctantly, than losing the battle and finding herself standing on the pavement some time today, choosing between driving away and leaving him when she's said he can't stay, or missing the ferry and cancelling the family holiday.
You're quite right it's outrageous, Cloud. It is. That's why some parents of teens like this get so desperate and feel so powerless and angry... But all the outrage in the world will not change the fact that, in this particular situation, the OP can't make her DS get into that car: he is only getting in if he decides he will.