This is the classic family pushback game, which sometimes is played by friends. I have tactics for you but with your SO on John's side, your odds are horrible. First defense, then offense.

His objective is to provoke you into what your SO will consider an overreaction. In private, fighting words. In front of her, sometimes microaggressions, sometimes insults. Your defense is to be unfailingly friendly, courteous and imperturbable.

To satisfy your SO that you're not exaggerating, if she'll cooperate: establish two code phrases (or silent signals) with her prior to each get-together. Code phrases that mean almost nothing: "no kidding" "you don't say" "really?". Silent signals like a touch to the hand or arm of your SO, or, if you first make eye contact, scratching your chin, for example.

The first code phrase will mean "I think I was just insulted - but you be the judge." The second code phrase will mean "I really need to leave - will you come with me?" Asking her about the time might work for that.

On offense, actively cultivate the goodwill of those closest to John who do not share his hostility to you. He's going to shop his opinion around, in search of validation, sympathy and support, and your objective is to starve him of these things, and even cause his own people to challenge him. This will take the starch out of his collar far better than anything you could say to him yourself.

Your SO's willingness to participate in either the offensive or defensive game is key. If she won't play, John will be YOUR problem, not hers, and you may just have to excuse yourself from further get-togethers.