yeah I would agree that doing a biopsy would be minimally invasive.
unless he has some health condition or something that is making them hesitant to do that?
and from our experience, your PSA score does need to be multiplied by a factor from being on proscar or avodart. But, we were on avodart for only about 4 months after diagnosis but pre-surgery. And it went down from 3.7 to 1.9. (but doc multiplied it to about 3.4).
I am not sure though if being on it as long as your dad and having pretty stable PSA's would mean it has stabilized? But anyway, is there any disadvantage in his case in getting a biopsy?