I wouldn't for one second attempt a diagnosis, but it was obvious from his media appearances prior to the Scotland job that he had some kind of illness. Primarily from the mild slurring in his speech.
Of course, that doesn't mean dementia - for all I know it could be a dental issue. Or even a mild stroke from which he's made an otherwise full recovery. Unfortunately, as a prominent centre-half in his day that spent many years heading heavy footballs, it is more likely to be some kind of head-trauma related condition.
Regardless, I agree with Al B. Putting his history as Motherwell gaffer aside, he is worthy of the legend status playing in a team that broke the OF stranglehold on the SPL title, winning a European trophy and being half of a defnesive partnership with Willie Miller that was truly world class. He just shouldn't have been anywhere near the Scotland job.