There are a number of things that could be going on with your cat - but trouble breathing really worries me! Any time an animal has difficulty breathing, he should be seen by a veterinarian immediately!
At first, I was wondering whether he might have an upper respiratory tract infection, as these are common in cats (it is the cat version of a cold virus in us). But with this, cats are usually sneezing a lot, and you mentioned that your boy is not sneezing.
With the further information you have given me, I am worried that your kitty may have congestive heart failure. In cat, this could be caused by a few different things, but hypertrophic cardiomyopathy would be the most common. With HCM, cats are born with this heart problem, or may develop it secondary to other disorders such as hyperthyroidism. Either way, the heart muscle is thicker than is should be, which means that there is less room witihin the heart chambers for blood. That is because the walls are thicker, pressing inwards. So, when the heart beats, there is a smaller volumer of blood ejected from it. The heart then has to beat faster to get the needed blood around the body. This is a lot of work for the heart, and it can start to fail. When this happens, not all of the blood inside it is pushed forward. So, now you have more blood coming in, PLUS that 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.
Here are some links to more information about congestive heart failure (CHF):
This disease is something which needs prompt veterinary attention. Your cat 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.
Blood tests might be needed to determine if this problem is due to hyperthyroidism, which is a very treatable disorder of the thyroid gland in which it produces too much hormone.
Here is more about Hypertrophic Cardiomyopathy which is the most common heart problem in cats, and the most common reason they develop pulmonary edema and congestive heart failure:
It sounds as though your kitty 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 realize that this may mean seeing an emergency veterinarian on this holiday Monday, but I do think you should do that since this condition is life-threatening.