Was the Complete Gleason score 3+3 ? Was there anything detected in the DRE? Based on just those few numbers and the PSA is 5.2 / 2 of 12 cores pos. then maybe you urologist is following sop to be safe. Some doctors will require CATs and some do not, but I have heard a PSA of 20 or above will prompt a CAT scan. The back pain is likely to be what your doctor diagnosed, and hopefully not kidney stones.
I wish your father good luck on the upcoming CAT scan, and please keep us posted on his progress. We will all be here to help you and your dad any way we can.