Excess mucus causes coughing. You can get a mucolytic (mucus thinner) called guaifenesin over-the-counter. Mucinex contains gauifenesin but I would get the plain guaifenesin. Vitamin B5 also thins mucus. He would have to take 500 mg a day. No side effects. Then there are anti-inflammatories, among them Vitamin C. Of course, stopping smoking would help the most, but you have had no success in that area. Maybe once he gets more comfortable at reducing the coughing he will think about quitting.
Hi!
Your father appears to have all factors that can aggravate his coughing. The only way to help him is to get regular check-ups done and to see that he takes his medications on time. Along with that maybe a counseling session may help him understand his problem. Keeping him busy in activities he likes can also help reduce smoking. You can enroll him into nay quit smoking programs as well. Take care!