A Gleason score of 6 according to many is most likely not spread. I have seen several articles that are asking if a Gleason 6 should be considered cancer. My Gleason was a 9 and my PSA was 2.3.
A cancer diagnosis is a diagnosis so it is wise to stay on top of it.
PSA scores are not indications of cancer they only measure how much of the prostate antigen is present in the blood at the time blood was drawn. YES ...PSA can rise and fall + many things can impact PSA levels - such as riding a bike, the size of the prostate, sexual activity, prostate infections and more.
What you will want to watch are the Gleason scores along with a steady rising PSA. I suspect your dad is being seen by a urologist which at this point may be far better than a GP. Ask questions and if you are not sure do not hesitate to meet with another urologist - preferably one who is connected with a regional medical center or university. Hope this helps a bit and please continue to reach out here.
Dennis(ProstateCancer.net TEAM)