This appears to be an upper respiratory infection. These are often due to a virus that is usually self-limiting (will go away on its own) - much like a cold with people. These can become worse and complicated with bacterial infections, so if Cooper is having issues with congestion, or starts to develop yellow/green nose or eye discharge, then it will be best to take him to a vet for an accurate diagnosis and treatment.
It's possible to ask your vet for a sedative to get him in the carrier, but most vets are very cautious about giving a medication before seeing a cat. Otherwise, the vet will be able to give sedation at the vet office if he's too difficult to handle. There are no over the counter medications or supplements that are effective for sedating a cat.
For now, make sure he is comfortable and free of any stressful situations. These infections often happen in times of stress, so if there was a potential new stressor, then try your best to remove it.
If he is congested, then you can do a steam bath at home. Below is a link to an article that explains this and has helpful info about cat upper respiratory disease.
http://www.wikihow.com/Help-Your-Cat-Breathe-EasierI hope that helps and Cooper gets better soon.