7 year old is really not all that old, your rabbit can still live for several years. And if he has a problem, he needs to be treated. What you describe is a lot too vague for us to tell you anything. Breathing how? When did it start? Is it something that he does all the time or only from time to time? Did anything change in the rabbit environment / situation? Is there any discharge around the rabbit's nose? Any other symptom? Is he sneezing?
If you feel something is wrong or that your rabbit is in pain you need to see a rabbit-savvy vet and fast. Rabbits can live for a long time, but when an illness sinks its claws into them, their state tends to worsen quickly. Consulting a vet without delay to pinpoint the root of the problem and begin a treatment is often the key to save them - if you wait, your rabbit can die from an illness that would have been easy to cure if you had brought him to a vet. And if your rabbit really has a problem with his lungs, it's possible he needs antibiotics.