My husbands coughs constantly throughout the night and complains of dry thoat. What can help reduce his problem?

Needs diagnosis... Common etiologies of a night cough includes gerd and post-nasal drip especially if it worsens when he lies down. Bronchospasm is a possibility especially if he has an allergy to something in the bedroom. If you notice pauses in his breathing, he could have obstructive sleep apnea. He needs to be evaluated by his doctor for diagnosis and treatment. He may need some tests to make this diagnosis.
Cough at night. A nightime cough that is not present in the daytime might be due to gastroesophageal reflux disease or gerd. The cough is from regurgitated stomach contents into the throad and around the larynx area. See your doctor for an examination and diagnosis to help you.