Zytiga (Abiraterone) - are these results good?

As to your father and his response, I think that it is an excellent indicator of activity. We never know if psa actually is tracking the disease, especially for the more advanced cases. It is often unused by trials which consider other bone and radiographic progression endpoints. However, a psa response of more than 80% is surely a strong indicator of activity.
As to the question of survival, the figure you refer to is from a phase three trial. This included many men who had no response or a transient response along with all others. The median of that trial was a four month survival advantage. In the case of your father, he is a trial of 1. The experience of 800 men (or some number) certainly does not extrapolate to any single man. Certainly not your father. The trial describes the effect in a large group of men and does not predict response in any individual man.
It is also true that survival is not linear, as you appear to suggest. A psa decline, to use just one measure, of 6 months before progression, does not mean a patient would live six months longer than otherwise. There is no proof for that. It is often found that after the tumor mutates and progresses it moves faster than before the successful intervention. It is now a different set of cells.
The point is that your father has responded as well as, or better than many of the men on the trial. This may suggest that his survival will also be greater than the median of the men on the trial. Enjoy today. It is a gift.