I hate to say it but I don't think you can .
Try to remember that he hasn't said no to you to be horrible, he loves you and he's being protective of his "little" girl as he's bound to still consider you. How about you try to get him to compromise with you? Ask him at what age you are allowed to have a boyfriend and if he says for example 15 ...... try to persuade him that you can be trusted to have a boyfriend at 14, you may have to spend the next year proving this to him but it's not really that long to wait. Of course if your Dad is anything like mine he might say 65
Other than that I'm afraid you are going to have to stick with what ever rules your parents lay down, it may seem harsh at the moment but they are only doing what they think is best for you.