The proof would be that he has dementia now and that dementia takes at least 4 or 5 years to progress to this point. So if he has dementia now he most certainly had it then.
Are you arguing he didnt have dementia then and I was wrong and he just suddenly developed it recently and the two are unrelated? That seems like some mental gymnastics honestly.
And we can say it was rapidly progressing at the time because we can see how severe it is now 4 years later. The only dementia that sets in that quickly or faster is mad cow disease form (months)... but in his case progressing this far in 4 years would count as the second fastest progressing type (lewy body demntia) at least. so yea, absolutely rapidly progressing.