Putting a humidifier near your dog's kennel or bed will help moisturize the air, which means that your dog's cough will be a little less dry and painful.
This can be done by placing a humidifiernear his rest area or by filling the bathtub with steaming water and letting the dog hang out in the bathroom for a bit (not in the water), just to let the steam loosen up his sinuses and lungs.