Is it possible he could have depression? How many hours does he sleep per night? Is he staying up too late? Or finding it difficult to sleep? People over sleep when they are very stressed or depressed. He needs to see a doctor again and possibly a therapist.
I have heard that depression can be due to a lack of serotonin in the brain. He could need medication if this is the case: http://serendip.brynmawr.edu/bb/neuro/neuro99/web3/Ho.html I suggest he gets his serontonin levels tested if there are no psychological or emotional reasons for depression.
I could be way off. He may just lack motivation and is lazy-doesn't want to work. If that is the case-I would tell him that this is his last chance. If he gets fired again, you are leaving and MEAN IT
"Don't ask a question if you can't handle the answer".