@workworkworkugh
Thanks for the advice so far everyone, it's massively helpful and I don't feel so alone in this.
I'm reading through all the replies and will try and answer soon, but to the few people who have suggested we try and take him away, that's nearly how all of this started.
We're going away for three days over New Years and he is refusing to come. He thinks he can stay home alone.
I'm not sure what we can do if he decides to take off to her house (with his key) and refuses to come, I'm assuming nothing 😔
I'm aware he's not coming off well and I fully know he's not an angel and it's not entirely his gf's fault.
We have always said we will pick him up from anywhere at anytime (parties etc) it was just a safety thing and we're happy to do it.
But for now the on call taxi service from us has stopped and the privileges are starting to be taken away, but that just means that now he calls the mother/father and they take him everywhere
Ok in response to the above.
Firstly it's obvious for whatever reason his GF's parents are enabling this situation and frankly massively over stepping boundaries.
To be fair, there is a possibility they actually may be doing this because they believe it's the right thing to do on the basis things your DS has told them (just think of the things your DS has said to you that you know not to be true or an exaggeration and how that would be perceived by someone else).
It's also possible that they are "cool" parents who fuel their own sense of self worth by living vicariously through their child.
Either way they are not going to be your allies and there is nothing you can gain by contacting them to try and get them "onside". Anything you say (however gently) will only be perceived as attacking them/their DD or "proof" that you are an over controlling parent.
New Years. No you can't force him to go away with you. He's well past the age where you can strong arm him into a car.
Your choice is go or not. If you don't go the rest of the family misses out and he'll still spend his time with his GF.
Realistically if he stays "home alone" he's going to go to his GF's house or she will come to yours. I'll bet good money he'll go to hers where he'll be fed, watered and fawned upon.
My response would be to let him stay at home and frame it as it's kind of him to "mind" the home whilst you're away. Give him responsibility for looking after the family house.
With that in mind I'd incentivise him/her to stay at yours. Stack the fridge with food. Leave money for a pizza takeaway. Let them download a movie of their choice etc.
He's going to be with her either way, so you might as well come across as accepting/kind about it and knock the wind out of his GF/GF parents sails.
Even if he still goes to hers you've been "supportive".
This is why you need to back off and be "neutral" about his GF (even uninterested). Anything you say that might vaguely be perceived as negative just fuels the flame.
Have faith. He's still your son and the love/values you've instilled in him aren't lost.
They're just covered in a sticky residue of hormones and the inevitable pushing of boundaries that are a prerequisite to becoming an adult, that will wash off as he matures.
It's like a game of chess. You need to think many, many moves ahead and tactically sacrifice some pawns to protect your most important piece on the board.