Could be loads of things unfortunately. You'll prob have to wait for opthalmologists opinion. Does he have any underlying illness like Diabetes or blood pressure? Could be (awful term) senile macular degeneration. This has various treatments depending on exact type/stage. It's also possible that it 's something he's had all his life, eg due to childhood infection, but hs only just been found.
What sypmtoms does he have? May sound like a dumb question but different probs cause diff patterns of visual loss.If they thought it could have been a tumour I guess he may have had tunnel vision?
Don't think this really helps I'm afraid. Hope it's something trivial.