Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Linus has labored breathing today and is unsteady on his feet.
I suspect his unsteadiness on his feet is due to weakness because he isn't exchanging oxygen normally.
A normal cat has a resting respiratory rate of about 10 to 15 breaths per minute, and one breath is considered the in and out motion of his chest. If he has a respiratory rate faster than 40 breaths per minute then he is in trouble.
I tell clients that if you can notice that your kitty is breathing abnormally then I am concerned for him, as in most cases cats tend hide being sick very well until they get very, very sick.
If he doesn't have a nasal discharge and isn't sneezing then his difficulty breathing is likely related to a problem in his chest or secondary to anemia (decreased numbers of red blood cells).
Problems in his chest leading to an increased respiratory include primary heart disease (including cardiomyopathy and heartworm), lung disease including asthma, bacterial, viral, parasitic (lungworm) or fungal infections, a mass(es) in the chest including lymphoma, heart based tumors or carcinomas, or fluid around the lungs (pleural effusion) due to a mass, bleeding, chylothorax or an infection.
Anemia can be secondary to poor red blood cell production, either due to kidney disease as the kidneys make a hormone to stimulate production or primary bone marrow disease, or anemia can be due to bleeding or destruction of red cells due to a tumor, blood parasite or autoimmune disease (body attacks its own red blood cells and destroys them).
If your fellow's tongue and gum color are a nice bubblegum pink them he is in better shape then if his gum and tongue color is blue or gray, these would signify he is real trouble and this is a true emergency.
The conditions I listed above are serious and I would highly recommend that your fellow see a veterinarian promptly to have an examination and further testing done based upon his examination. Linus likely needs radiographs of his chest to evaluate his heart and lungs to start and then further diagnostics based upon those findings. That may include testing for heartworm and lungworms, aspirating fluid for examination if there is fluid present around his lungs, or blood titers for fungal infections.
Unfortunately this is not something that can be helped at home but in the meantime keep him quiet so he can breathe as easily as possible.
Please let me know if you have any further questions.