Having raised a daughter who was dying from heart disease and was transplanted by the age of 22, I see nothing in what you wrote that makes me think your husband is dying. His depression alone can cause him to be tired all the time; people who are depressed tend to sleep a lot and the problem is well known for having that symptom. He needs to get help and if he won't call the doctor, you should do it for him. Once a person starts suffering from depression, the chemicals in the brain start to become unbalanced and it takes medication to get things back to normal. Ed24 is absolutely right about the fact that if things were that bad the doctors would have had your husband evaluated by a transplant team. Most transplant teams place a patient on the list when they feel that patient has approxiately 18 months of survival left; that's how long it takes, generally, to secure a new heart. It sounds more like the depression is the real issue here; try and get him onto something and he'll probably feel so much better.
After 5 bypasses I think I would be depressed. I actually have great admiration for your husband for putting up with so many operations. Many heart disease patients have a natural feeling that they only have a short time to live but many go on to live long healthy lives. It is psychologically hard on a person due to the constant symptoms such as tiredness etc, many of which are due to medications to prevent the heart from over working. It is not possible to put a time on someones life with heart disease. With illnesses like cancer it can be a bit easier in some cases. Research is continuing into heart disease day after day and new discoveries are being found all the time. It was only recently for example that a gene was discovered that is a culprit for causing heart disease. This gene was found in dogs, and was then found to exist the same in humans. Stem cell research is flying along now, animal hearts are being grown in labs and fully functioning, so human trials will probably be in the next few years. This would mean a transplant of a new heart made from cells taken from your own body, with no rejection.
There is a lot of light in the seemingly narrow tunnel, and there is no reason why your husband has to think the way he is. If he forms more blockages, these too can probably be stented. Maybe you should have a chat with his GP and discuss the depression. I know for example when I was on beta blockers I felt quite depressed because they prevented me from having the energy to do the things I wanted to.
The way I see it is that if his cardiologist feels all hope is lost, then why hasn't he been tested to see if he can be put on a transplant list? They must feel theres a lot left in his heart yet.