What Causes Persistent Cough With All Tests Being Normal?

Hi, my husband has had a cough for over a month. He went to his doctor who sent him to the emergency room. He had an EKG, blood work and a chest X-ray. Everything came back normal so they sent him home with no medication. That was a week ago. He isn't getting worse but every couple of days he coughs. It just isn't getting better. Not sure what he should do.