Hi boden
The advice to screenshot the conversations is very good.
It means when you shut him down for the final time if he turns nasty you have evidence to show the people you need to like his sister for example.
I know this is hurting you as you are close to the whole family and he is taking that away with his behaviour and I'm sorry for that.
You do need to set firm boundaries from now though, make it clear that you don't like this type of conversation with him, you see him like family and it wields you out.
I know that you have probably tried to play it down with him and in your head because you care for him and can't believe this man who you have known so long could actually not be the person you thought but all the evidence points to the fact it is not in your head and he has turned out to be not the man you cared for (in a friend way) all these years.
He has left you no choice but to be like steel, unbendable solid and like a wall unmovable.
to make it clear I am not saying any of this is your fault
All I'm saying is from now you need to have a clear head and shut down the friend feelings when dealing with this because actually he is not being a friend to you!