The most extraordinary thing that I don't understand about The Script is how, how on earth, do so many men follow the same Script when they have presumably not been taught it school? Surely it isn't a hidden lesson or exam at school that all of the boys keep to themselves?
It's because there are only so many ways you can manipulate somebody when you're caught cheating and don't want her to leave you. You can deny it and just keep lying and gaslighting, which is what OP's partner appears to be doing, you can admit to it but blame your cheated on partner for it, or you can admit but swear the OW meant nothing, that it's over, and beg your partner to stay. This last technique is when the crocodile tears come out.
Equally, there are only so many things you can say which will facilitate these manipulation techniques.
That's why there seems to be a script they've all read.
OP, as other posters have pointed out, he is gaslighting you, and it does not matter how overused the word is because this is very definition of it. You saw some evidence that adds up to cheating and he's convinced you that you were being irrational to think the dodgy messages actually were evidence of cheating. He said he was going on a date and needed an "upgrade."
That sounds pretty conclusive, unless he was talking to a friend and was joking. Even if he was, it's a mean joke to say he needs to upgrade from you and is pretty revealing as to how he sees you. Also, why would he not say it was a joke instead of saying it meant nothing?
Those are really the only two possible interpretations, so
however you look at it, this is shady AF.
Stop letting him talk you into thinking you were in the wrong.
As for telling his relatives, this is another classic cheater techniqe. They spread the word that you are irrational so that nobody will believe you even if you have unequivocal proof of cheating. That is also part of the script- the smear campaign.
By cheaterspeak I meant the typical bullshit they come out with when they are caught.
When my ex was cheating he made sure to lie about me behind my back so that he would have people's sympathy if his deceit was discovered. This is also in The Script. It worked so well that some of my own family members were on his side. One of them, possibly two, still are, all these years later even though he cut off all contact with them.
So I'd talk to your family and friends about this before he grabs the narrative with them, too.