Do you have other cats right now? I'd keep him isolated from them until you can get him to the vet and get him tested for FIV and Feline Leukemia. That he is active is a fine thing, I would call the vet and ask if you can bring him in this week, and pay next week, just to be sure he's okay, and get him something to help with his respiratory infection if that is what it is. Have you checked him over for fleas, etc.?