It doesn't work like that. The prostate can stay the same size but the cancer can be spreading in the bones or lymph nodes.
You fix too much on individual things instead of asking the right questions of your dad's doctors. The PSA isn't the actual problem, it is an indicator of how active the cancer is. The pain isn't the actual problem, it could be an indicator that the cancer is active. The size of the prostate isn't the actual problem, it is a side effect of prostate cancer that can make urination difficult. But one thing does not automatically mean another and nor can one thing be used to rule out something else. Your dad's doctors should be using all of the information to monitor him and judge whether the cancer is being controlled.