Don't worry at all about what he is telling others op. Anyone with half an ounce of common sense will interpret his pronouncements as immature bluster. The thing that is so exasperating though is that they themselves really believe it to be true!
There is no arguing with it imho. Their brains aren't fully developed until they are 25 years old and they can interpret thingsselfishly differently to us. I read somewhere that this is a crucial developmental stage which allows them to form their own individual characters. Some teens literally imho cannot see things from another person's pov. Or rather they will feel global injustices such as the plight of refugees very keenly and not see the irony when they come home and complain when you haven't cooked their favour dinner!
My teen is prone to mouthing off saying very unpleasant things to us and then will wander back in the room two minutes later and ask to borrow the sellotape as if nothing had happened. We are like 
what just happened?!
I must admit we also have problems with the teen "negative filter" so when I say to my daughter "we are concerned that you are spending too much time on your phone" she hears "we hate you, you are a despicable person, we want to ruin your life". She also does the entitlement/over-optimistic assessment of her own contribution, having hoovered once for 20 mins in the last month (she's actually done much more than that recently but I'm using a past RL example).
I pick my battles but very occasionally I find it helpful writing my daughter an e-mail, trying to be as fair as possible, writing down why I said what I did, and how she responded. I always emphasize how much I love her (as it seems to be some teens default position that they are hated by everyone). Sometimes she replies, sometimes she doesn't. It's an interesting exercise.
The thing here op is to look at the long game. Parenting is a marathon not a sprint. These are all "normal" developmental quirks and he will come out the other side eventually. In the meantime you have to step back a bit, let him separate himself from you to an extent, but don't let him get away with all the fun bits of being more "adult" while shirking the responsibilities. So, for example, if he chooses to reject you and live somewhere else other than home, then you are not as available to help do his washing, drive him around, help him generally. Maybe he needs to think about giving some of his allowance to his hosts? Maybe not be as available answering his calls for lifts?
I also read somewhere that those teens most attached to their parents have the hardest fight (emotionally) to separate themselves off from them in order to become an individual in their own right. This might be the case for your son if he is very bonded to you. There are several ways teens separate themselves off from their parents: spending long periods alone in a bedroom, extreme bonding with friends, drugs and alcohol, or, in your case, literally moving away in to their gf's house!
It's painful but try and step back emotionally too. Get on with your own life. Don't be as emotionally available but emphasize you love him and are ready to talk at any time. After that, step back a bit. Don't allow yourself to be driven to distraction by him (although I recognise the feeling of shock and bewilderment when hormones "steal" the compliant, helpful, child you recognize and replace them with a defiant, ungrateful, ball of anger). He still needs you to be the stable rock in his life that he can kick against and launch off in to the world, even though he has you tearing your hair out currently.
I'm afraid you need to play act a lot as a parent of a defiant teen so even when you are shaking with inward rage and want to shout "you ungrateful know-it-all little sod" we must try and breathe, keep calm, tell them you love them, and let them make their own mistakes to a large degree, and act as back up support from a distance.
[If anyone who is reading this is horrified and thinks this interpretation is very unfair to teens then rest assured it is quite possible as a parent to love and admire and respect your teen with your whole heart, and appreciate all of their good qualities, while simultaneously feeling completely frustrated and overwhelmed by their mood swings, obstinacy and general chippiness. And if you think "gosh my child and I have a marvelous relationship and he/she would never behave that way towards me" then hang on to your hats because I thought that too! And if you have perfectly delightful helpful and compliant teens then bravo! Hats off to you!]
Sorry for the essay. Good luck op 