You're in a very painful situation. I have to assume that this guy stopped dating you and began seeing someone else precisely because of your father.
If you really feel so strongly about this guy, then you need to talk to him and find out if it's true that he's only dating this other girl because he can't be with you. You need to be sensitive to the fact that there is currently another person involved in this equation.
The woman that he's seeing has feelings and this needs to be considered. She may love this guy and we don't want to crush her in the process.
If the man you love tells you that he doesn't love this other woman, then staying with her just because he can't have you isn't being kind to her in the long run. If this is what he tells you, then you both need to decide if you're willing to face your father head-on.
In the end, it's your life, not your father 's. Your father needs to be told that he must support you in your choices. That's what a good father does! He doesn't have to love your boyfriend, but he does have to be polite to him and respect your decision.
Instead of turning all the power over to your father, realize that you have power of your own.
You can also choose to disinherit your father for his bad behavior. This is your way of saying that the knife cuts both ways and you can cut your father out just as easily as he can cut you off.
If your father gets a taste of his own medicine, he won't like it. If you and your boyfriend are a united front and not willing to be bullied, your father will have no choice but to eventually come around.