My Father Has Been Coughing On And Off. He Is

My father has been coughing on and off. He is asthmatic. The cough doesn’t happen all day. At certain times it can become heavy and it a nonstop cough. At night when he trying to sleep the coughing comes up again and you could him groan. He will groan and start coughing. Eventually he will fall asleep and the coughing and groaning stop. What do you think it is ??