Indeed, the brain is very good at "filling in" holes in its senses. But in this case even after it was pointed out to him that he was missing part of his vision he didn't seem to be capable of acknowledging or adapting to it, so it was a bit more than it just not being obvious. He seemed to be unable to comprehend that losing that part of his vision was something that was even possible. I suppose his brain was "filling in" more than just the hole in the vision itself. Since the damage was to the visual center of his brain rather than the eyes themselves this seemed like an understandable manifestation.

"Fortunately" by the time that happened he'd already become unable to walk on his own, so he didn't end up crashing into stuff or otherwise having accidents. We just had to make sure to put the things that we brought to him over to his right side, where he could see them and interact with them more easily.