If he hasn't been on heartworm preventative, then a heartworm test needs to be done to rule out cough due to heartworms.
So in a cough in a dog this age, one of my first suspicions would be chronic bronchitis. Here is a detailed article on this disease:http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=3151
And I worry about heart disease causing a cough. This is why it is so important for him to have a vet examine him. We need to find out if he has a heart murmur or not, and ideally he should have a chest x-ray done.
For chronic bronchitis, it is often impossible to completely get rid of the cough,but we can often see drastic improvement. Initially depending on the severity of cough these guys usually need to go on a combination of steroids (that is the prednisone), bronchodilators (that is the theophylline and albuterol) and cough suppressants. We will also often do a trial of an antibiotic to ensure there isn't a secondary bacterial component and sometimes these antibiotics have anti inflammatory properties which can help.
If the cough is due to heart disease, then there are some oral medications that can help, such as lasix (people call these water pills sometimes), and other heart medications depending on the severity.
The other thing we worry about in elderly dogs that cough would be tumors or growths in the lungs. The only way to detect these would be a chest x-ray.