You asked what "small volume" meant - your father should ask his urologist.
However, I will hazard a guess. Typically, each core is scored as to both the Gleason score (meaning the degree of cell alteration from normal) and the percent of the core that has cancerous cells.
As an example, I had 4 of 12 cores that had cancer - all were Gleason 6 (3+3). However, one was 40% cancer, one was 10%, and the other two were "small focus". For me, this is a preliminary indication of where the cancer might have started (near the 40% core) and that it is spreading (although note that prostate cancer is very slow growing). Of course, we will not no for certain until the prostate is removed (next week) and the whole gland is send for testing.
By the way, you shour read (if you haven't already) the earlier message titled "Gleason Score" - it is really good.
You will see lots of threads here with men after various treatments for prostate cancer and doing really well. Take heart, there is plently of hope ahead.