Dear Diane,
I'm sorry to hear about your disappointment.
I'm not a medical professional, but here is what I understand from what I read so far: When they measure how well your kidneys are functioning, they then have to calculate how well you will do after losing HALF of your total kidney mass by donating one kidney. The remaining kidney will work harder to try to do the whole job, and may increase in size a bit. But if with both your kidneys functioning it is just enough, the worry is that one kidney will not be enough for you. Likewise the one donated kidney may not be strong enough to do the full job for the recipient. They also have to take into account that everyone's kidney function tends to decline with age. So even if your remaining kidney were sufficient for you at present, it might become insufficient later in life. The doctors do not want you to donate a kidney now and then end up on a transplant waiting list yourself later. Nor do they want to transplant a kidney into your dad if it looks like that sole kidney will not be able to do double its work.
It can be a difficult decision, especially if the numbers are "borderline".
It would be good to make sure that other relatives, and your dad's friends, co-workers, church/club members, old alumni, neighbors, etc. know about his need and about the possibility of living donation. There may be other potential donors around, including people of his same age, who are not aware of his need.
best wishes,
Fr. Pat