My Dog Is Coughing And Very Lethargic , What Is Wrong?

Most kennel cough infections are mild, self-limiting, and respond to medication.  Some dogs have a more serious case and need more aggressive treatment--fluid therapy and intravenous anti-biotics.  It is also possible that the infection is now in the lower respiratory tract (lungs) and your dog has pneumonia.
Your dog needs to be re-checked by your veterinarian and started on more aggressive therapy.  Chest x-rays should be done to rule out pneumonia and visualized the heart silhouette.

I would seek veterinary advice. If your dog is coughing and very lethargic it could be that he has a heart problem. The heart doesn't get enough blood and thereby oxygen, round to all the organs, the lungs, the brain. This will make the dog very tired indeed and out of breath, which also causes coughing.

He need to see the vet. It could be that he got into something yummy and has really upset his digestion or it could be that he has caught a virus. A knowledgeable person needs to be able to give a physical exam.