Ok, seriously, what is his hemoglobin reading? How old is he?
Until your last post I was starting to lean toward early onset Alzheimers which would include some memory loss and lethargy...basically a lack of initiative to do anything which could be confused with depression. Especially if it's a mixed dementia. Given his cardiovascular history it is a possibility that he had vascular dementia as well. A few well placed questions and interrogation would help to identify this. Asking him to identify things such as his friends or items in the house would help. Also, asking him to remember three things, then recall those three things later on would help to assess his memory.
Now, following your last post I am interested to see if there is any anemia occurring.