trazzy, it's so good that you are taking close care. The MRI is a good measure it won't measure TIA, unless it recent, but brain bleeds tumors and structural abnormalities
Never hurts to see a geriatrician.
I would want a second from an oncologist. There have been many changes in prostate cancer care in the last five years with checkpoint inhibitors and a new contrast agents for detecting spread. Our brains are often a destination for metasatic cancers. I pray not and it is good that the oncologist thinks not, but it is something that you'd need to know.
On the other hand, sometimes the elderly are too quickly slotted into a dementia diagnosis. Depression is a cause of fatigue. And fatigue on its own can cause lack of concentration which causes loss of balance and seeming hearing loss.
Low testosterone usually causes some fatigue and depression. I can't think that low dose testosterone gel would be restorative.
From my experience aftercare for cancer isn't great, especially after the five year mark.
You might take him for a hearing test. It is easiest at the hearing aid retailers, though you could see an otolargyngologist. I would want to test testoserone levels, if they haven't been tested.
We drive ourselves crazy speculating. I would schedule both doctors at right away, so you aren't waiting so long. You probably don't need a referral for either.
I'm praying that it's low B12.
Best wishes