Does rapidly declining memory mean Alzheimer's disease or vascular dementia?

Hello doctor,

My father is experiencing rapidly declining memory and cognitive function. He was taken to the hospital a year back due to confusion and memory loss and was treated for diabetic ketoacidosis and had a quadruple bypass performed. They also did various scans and magnetic resonance imaging (MRIs) and found nothing to indicate a reason for the memory issues. They ascribed it to the diabetic ketoacidosis (DKA) and lack of oxygen to the brain due to heart blockages and told us his symptoms should subside after his recovery. After 13 months, he is only getting worse. He cannot remember appointments or plans and has to put everything on his phone to remember (including his birthday). He does not remember what he has done the previous day and sometimes believes he did something he did not. He does not remember events of past 10 to 15 years (including forgetting where his house is that he has lived in for 24 years), but his youth is still vivid in his mind. He turns 61 tomorrow, so he is fairly young for this kind of memory loss. Does this point to Alzheimer's, and if so, what stage is he in? Or does this seem more likely to be vascular dementia? Or something else entirely?