At what point do you have to step in especially when they don't think they need you?

My dad has dementia and his memory is very poor, when he thinks he remembers stuff, but then make thing up, which he maybe partially right. It worries me because these thing are usually financially related and I'm afraid he will make a big mistake or spend money where he shouldn't, call someone he shouldn't and piss them off with wild things he has in his mind that to him are true or real. I catch him saying thing that aren't true but avoid pointing them out because it ends in a argument. Even if I'm 100% right his mined is set on what he thinks. Even if I'm prove it, he will forget and it will just start over again another time. Money is the biggest thing on his mind, he is very concentrated on it. And for me (his son) he doesn't full trust me with financials I think. Even though when he was sick and dying I did them all for months till he recovered. He insist on doing thing himself and it kills me to see him struggle or screw it all up. This is just getting harder and harder as time goes on, seeing th mind slowly die is very difficult to deal with. I thought I could wait till a big mistake was made and I would have to step in, but I'm fearing that more and more. And if I can't convince him of the little things I have no chance at the big ones.