Why can't my dog stop coughing?

He is a blue heeler mixed with lab (I think). He is about 5 years old, around 70 lbs. He has developed a really bad cough and he seems to do it all the time. When I got him from the humane society, he was heartworm positive, and I had that treated (about 4 years ago). I had him tested again recently, and he came back negative. The vet prescribed some antibiotics for what she thought was a lung infection. I've been giving them to him but they aren't doing anything. Is there anything else this could be? He starts coughing when he gets just a little bit wound up, and he sometimes goes on for ten minutes straight. I feel terrible cause I want to help him, but the vets around here seem like they don't know what they are talking about. Please help. I love my dog to death, and I want to help him. What should I do?