This isn't just dementia. This is frontemporal dementia, AKA Pick's disease. Likely he's been suffering for years before it got bad enough to seek help. You don't even notice it at first. It starts with personality changes over years. They become a different person before they progress to having trouble speaking. By the time he became aphasic, the person he was had likely disappeared completely. Before anyone even realized he was sick. I feel worse for his youngest children, who likely never got to meet the real person before the disease.