Sorry to hear of your fathers problems and hope all comes out for the best.
I assume he has had the results of his MRI now, but with low PSA, clear bone scan and, though a higher Gleeson score than you would like, he seems to be in a strong place for successful treatment options.
I too was diagnosed at the age of 58, Stage T3a, PSA 13, Gleeson 7 (3-4) and 7 out of 10 samples came back positive (70%). Finally went with HDR Brachy combined with 6 months hormone therapy and 23 doses of external radiotherapy afterwards.
Almost 2 years on I have not had any incontinence issues at all, erection disappeared after 4 months but returned after 15 months with help from Viagra and pump. Pump no longer needed and Viagra down to 25 mg from 100mg now.
I am still working, get much more tired these days, but only later in the day. Other than that seem to be pretty fit. My PSA is slowly dropping, taken 2 years but now finally at 0.1 (Feb 2019).
Good luck and hope all goes well for your father.