Hi, I'm a licensed veterinarian and I'll be happy to help.
I certainly can understand wanting to avoid the stress of bringing him to the veterinary clinic, but I do not think it is a good idea to try to treat this with any over the counter medicines.
When I see an elderly cat with fast breathing, I usually suspect Pleural Effusion, which is a buildup of fluid around the lungs. This can go undetected on a routine physical exam, until the fluid fills to the point where it reduces how much air can fill the lungs.
Over the counter medicines such as antihistamines (Benadryl), would not be of any use in this condition, and if there is also any problem with the heart (which ALSO can go undetected on a routine physical exam), the benadryl will increase the stress on the heart.
I really must urge you to listen to the warning signs that Jacob is displaying for you. A chest x-ray is necessary as soon as reasonably possible. If there's pleural effusion, it needs to be drained to allow the lungs to inflate properly.