There is absolutely *no* Alzheimer's medication that can halt a patient's progression in a clinically meaningful way. I love the idea of him clinging to false hope, though, as his mind erodes.
(The best drugs for Alzheimer's are capable of slowing the progression of a single symptom, memory loss, by six months. So if he started on them at 78, then by age 83, he'll have the memory he would otherwise have had at age 82.5. The effect is so small it can only be detected in population studies. There's lots of trials, but there's also lots of grandiose claims that never go anywhere)