Your dog can be coughing for a number of reasons. A cough can be a sign of a serious underlying problem like kennel cough, distemper, coccidiosis. It can be a symptom of internal parasites, heartworm disease, distemper, fungus diseases, allergies, and pollutants such as cigarette.
This link is a guide to canine coughs:
http://www.canismajor.com/dog/cough.html
You should definitely bring him to the vet, this does not sound like something that is going to go away on its own. And it sounds as though it is getting worse.
Let me know if you have any other questions and also how your dog makes out.
Peace,
Christina
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Peace and happiness to you and your pet! ~ Christina
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