I have been on Zytiga for over 2 years, on a Clinical Trial for Patients who have not undergone Chemo, I do not know if there are any Trials accepting new participants, but other than being in an Insurance scheme that cover the full cost of treatment, the only other avenue, would be to apply to Johnson and Johnson for treatment under compassionate grounds.
I live in Australia and Zytiga has not been approved for treatment and I cannot see it happening any time soon, at least you are able to obtain this medication, but at US$5000 per month, I guess it is well out of reach for the average person.
As you maybe, aware Zytiga is not a cure, it only offers an extendsion of life, in some people, I am one of the lucky ones, that it has worked for and is still working, unfortunately the are many patients that it has not worked for and they have dicontinued useing it.
I wish you all the luck in the world, that it will work for your Father and you are able to find a way of obtaining this drug in a way that you can afford.