You say that dad has a heart of stone; is this the reason that he's denying you? I.e., no GOOD reason?
If he's got a heart of stone toward you, I extend my sympathy to you. In that case, all you can do is see what will float his boat, as mentioned, your doing a bunch of extra work for the privilege. And IF he has a softer heart toward horses, I'd approach it from the "horse welfare dept.": they need exercise, they need dry footing so that they don't break a leg outside, they're bored, they need the mental stimulation of playing in the arena, etc.