A dry, hacking cough that sounds like something might be stuck in the
dog’s throat is one of the symptoms of Kennel Cough. Sometimes this cough is so hard that the dog will bring up white phlegm; also, coming in from outdoors (changes in environmental temps) may trigger a coughing outburst.
Bordetella Bronchiseptica has an incubation period of 2 to 14 days. It’s often caught while dogs are boarded in kennels (hence the name) or even in dog obedience classes, dog parks, vet waiting rooms, etc..
Pet store and shelter pups are especially vulnerable. Bordetella B. is an airborne infection that’s transferred when dogs sneeze or cough and project droplets into the air in the vicinity of another animal. Direct contact between animals or sharing food/water bowls and in some cases when a dog comes upon an area that an infected dog recently visited may transfer the infection. This bacteria, in normal situations, isn’t especially difficult to treat. There is usually a secondary infection (or more) that needs to be addressed in order to eliminate the problem, so a vet check is warranted. The treatment is an antibiotic medication that may be given with a cough suppressant (a med that is prescribed )
Over the counter cough suppressants are not known to actually work in dogs or humans http://www.libertypost.org/cgi-bin/readart.cgi?ArtNum=124293
In dogs with immune system or other problems, this could be a more serious condition and indicate emergency treatment to insure a better chance of survival.
Other causes for dog coughing are environmental, like cigarette smoke exposure, dust, temperature extremes (too hot, too cold) and even stress. It could be tracheal disease or just allergies.
Even though the cough suppressants aren’t known to really work, some people still recommend Robitussin DM .5 to 1 mg per pound. There are eye dropper/measuring delivery systems available for baby care in most pharmacy aisles of stores. They’re great to have on hand for pets.
Now with all of this said, the symptom of lethargy and most importantly, going off food is serious. Not eating is often indicative of pain and no one wants their pet in pain. Finding out exactly what's going on is essential in order to eliminate the cause and not just treat the symptoms, right?
This is your call though. You can try the suggestions here and give it a day or two or you can bring him in now before it becomes more serious and thus, more costly and complicated to treat or even run tests for.
Let me know how you make out ok? BTW, EXCELLENT job in feeding and care. I really mean that