Rapists and paedophiles don't change.
It is heartbreaking for you that your son is taking this line and, more still, changing in the way he is. Very sadly, but honestly, that may mean you take a position of loving detachment for your own psychological safety & wellbeing.
If your son, in his ignorance and desperation to win his this man's approval, decides to parrot that you're 'bitter', so be it.
He is trying to heal the wounded child within that is desperate for his father's love & approval. The narrative he creates - taken from this man - allows him to have a normally-dysfunctional family, the parents who split and have an antagonistic relationship. Factor in that he is probably told how like his father he is in terms of charisma and...
There's no straightforward resolution here. I urge you to seek therapy to help you deal with the shifting relationship with your son & the trauma it both triggers and, perhaps, creates.
I am sure your son is a good person and will remain so but you may need to accept that for the foreseeable, you need to step back.
I believe you should tell your son the extent of this man's abuse.
- Tell him that men like that don't change. Say you understand how incredibly hard it is to hear and that his reaction may be one of furious denial and anger towards you.
- You will not stop him having a relationship with this man and he does not need to choose between you but you will not put yourself in a place of danger, even for your son's wedding.
- if your son needs to understand more, say you will answer his questions and explain as much as you are able but there may be a limit, a time where you have to stop for your own safety. Point him to some resources online about charismatic abusers & the long term trauma women experience that he can read when he's ready
- say his father is not worthy of his son and that whatever surface similarities there may be between them in looks, charisma, etc, your son is not his father. He is a better man, with a different character
- recognise that your son may interpret what you say as an attack on his father & therefore on him too - clarify that this is not the case
- tell him what you expect of him. From what you've said, it sounds like you're not asking your son to do anything but understand & respect your position and that you're not asking him (or expecting him) to reduce contact with his father or choose between you both
- reassure your son he is not like his father and that you are not rejecting him, and will always be there for him but what he is asking you to put yourself through is impossible
- say you understand how very hard it is and you hope that one day he will be able to understand
- pre-empt his father's assassination of your character in a calm manner. Explain that you realise his father might believe that you are bitter, unstable...whatever, but these are the facts and your position is a natural & normal one.
- reassure him of your love for him & his fiance
You cannot control your son's reaction, but you can control what you do and protect yourself. You have loved & protected your son his whole life, trust that this is enough to win through in the end.
I'm afraid there will be a fall out, but you absolutely mustn't put yourself through being in the same place as this man. Put your safety first now.
I suspect your son sees an opportunity to create the family he never had - two good parents, a wife and all the family she brings with her - and a happy, successful 2.4 children family of his own. So it might be that he cannot accept what you say...Hopefully he will in time.
One thing to add: if you force yourself to go and play happy families, you tell your son two things:
- that your ex was never really abusive to you
- that abuse & rape is not that bad
Don't minimise it. For his sake as well as your own.
And seek decent therapy support for yourself while this is going on xx