With an 8 year old dog there are several possibilites for the cough:
An upper respiratory infection (kennel cough) may be the cause. This is many times caused by a virus, and can also be caused by the Bordetella bacteria. The cough can last several weeks.
A heart problem could also cause coughing. Poms do get heart murmurs, and this sometimes leads to congestive heart failure and coughing.
Heart worms could cause a cough also. If you live in an area that has a problem with heart worms, this could be a concern.
Lung problems like pneumonia, bronchitis, cancer, etc., may also be the cause of the cough.
If your dog has ever had a history of a collapsing trachea, this may have suddenly worsened.

With so many possibilities, my recommendation to you is to bring your dog into the vet for a check up. They will listen to the dogs heart and lungs and determine if any tests are needed to help make a diagnosis. I would plan on xrays, but your vet may not think they are necessary.

The treatment of the cough depends on the cause, so you will need to find that first and then go from there. Your dog may only need a course of antibiotics and cough suppressants, but there certainly might be other things he will need as well.

Let me know if you have more questions.