Nat there are two words that terrify us - advanced and aggressive. In the case of prostate cancer, aggressive does not mean the same as advanced. So a Gleason 9 would be considered 'aggressive' - as Barry has explained, the cells are very distorted - but it doesn't mean that the cancer has 'advanced' or spread.
In your dad's case, they can already tell that he has an aggressive cancer but it may still be very small and fully contained - they won't know until they do all the scans. Conversely, sometimes men are diagnosed with a very low Gleason score but the tumour has spread extensively. There is no way of knowing until you have all the results and 'guessing' will simply upset you all.