Oh, there is plenty of circumstantial evidence, but nothing that would constitute proof without access to the results of a detailed medical examination. Source: watching the decline of family members, and others in the care home my mother is currently in.

The increasing randomness and apparent lack of concentration, the “resting his eyes” in some meetings, the leaning, etc. A lot of the signs could be other things of course, like just plain ol' age related decline. But if the people close to him don't at least have concerns, would he have been subject to the cognitive tests he is so proud of “winning”?