Thanks for that additional information.
I'm quite worried about your little dog and feel that his symptoms of coughing, difficulty breathing, raised head when breathing, distended belly and lethargy can all be explained by heart disease and congestive heart failure.
Here is a video about coughing in dogs, explaining that coughing can be a symptom of many different underlying problems – some quite trivial and some very serious.
There are a number of things that could be going on - but his symptoms really worry me! Any time an animal has difficulty breathing, he should be seen by a veterinarian immediately! The slow walking could well be because he is not getting enough oxygen to his tissues.
A common reason for laboured breathing is congestive heart failure.
In an older dog, this could be caused by a few different things, but mitral valve disease would be the most common.
Basically, when the heart beats, not all of the blood inside it is pushed forward as the mitral valve often becomes "leaky" in older small breed dogs.
So, now you have more blood coming in, PLUS that leaked blood that didn't go out, so the pressure builds up inside the heart. This then backs up so there is increased pressure in the blood vessels in the lungs. Because of the increased pressure, fluid gets squeezed out of the blood vessels and into the lungs. With water in the lungs, it is very difficult to get oxygen across the lungs into the blood stream.
Heart failure can also cause fluid to build up in the abdomen. This fluid is called "ascites" and is another symptom of heart problems. It sounds like your little guy has this.
Here are some links to more information about congestive heart failure (CHF):
And a great video explaining it:
This disease is something which needs prompt veterinary attention.
Your dog may need injections of a diuretic (such as Lasix/furosemide) to quickly move the fluid out of his lungs so he can breathe more easily. The vet would likely get x-rays of the chest to see how much fluid is there, and what size his heart is.
Once he is stabilized, he would likely go home with Lasix in pill form so that the fluid doesn't build up again, as well as something called an "ace inhibitor" to help the heart function more effectively.
It sounds as though your little dog is having a rough time right now, and it must be distressing to him to have trouble breathing. Please get him to a veterinarian as soon as possible! I would strongly recommend a visit to an emergency veterinarian tonight.
If that is absolutely impossible, please know that there is some danger of sudden death with this disease. At home, all you can do to help is to prop your little guy up so that he can rest with his head and chest higher than his belly and back legs. Having him on a wedge made for babies with colic, or on some other type of wedge to elevate his front end might help him to survive the night.
He really does need to see a vet as soon as possible. I hope he feels better soon!