I am sorry to hear that your fellow has been on his bed for the past 24 hours, breathing shallowly, and now has urinated on his bed and yours without bothering to get up and move away.
While he is not showing obvious signs of pain and illness such as crying out or vomiting I suspect that he is very uncomfortable and weak. Cats that urinate and lay in it are very, very sick as cats are fastidious animals. Although this seemed to come on quickly it is likely that he has been sick for awhile, but has successfully been hiding his illness from you.
Cats, being small creatures that tend to be secretive even when they are well often don't show obvious signs of pain, especially with a chronic, progressive, old age type illness such as organ failure. But his sleeping more is a way of hiding from his discomfort and is a way to try to conserve calories. Because once he stops eating completely his body's metabolism turns to starvation mode.
He likely feels weak, may feel nauseous and his mental acuity will begin to suffer, both due to low blood sugar and the build up of waste products due to his organ beginning to shut down and metabolites from muscle and fat tissue being broken down to keep up the caloric needs of base living.
This may be related to diabetes, but any illness which causes severe weakness (organ failure, heart disease or cancer) can cause what you are seeing.
This is not a pleasant way for him to pass. It can take a long period of time, in some cases weeks, and while he won't be in acute (sudden, high intensity) pain he will suffer a lingering feeling of feeling horrible. Imagine what you feel like when you have a bad case of the flu. You feel extraordinarily tired, your head hurts, you feel nauseous, dry mouth, your muscles ache and if you are sick long enough you may get delirious. That is how things will be for him. As his liver breaks down fats for energy to support brain and heart function it may fail, and this can cause altered mentation and possible seizures due to waste products accumulating. Cats that do not eat for more than 3-5 days straight run a high risk of developing this type of secondary liver disease (hepatic lipidosis).
In short I can understand not wanting to put a senior fellow through extensive testing and treatments which make him miserable, and do not contribute to any sort of improvement in his quality of life.
But there is a middle ground. Perhaps some simple blood tests and a urinalysis will tell you why he is the way he is and give you a reasonable prognosis for him to return to a good quality of life. If you believe that he will not tolerate any treatment or he is very sick and will not regain a good quality of life then humane euthanasia can be done.
I know he is a very well loved fellow, but letting him go as he is will unlikely result in heartache for both of you as he unhappily lingers.
Please let me know if you have any thoughts, questions or concerns with what I have written.