Hello and hope you are doing well.
Understand your predicament. Hope Asthma has been ruled out, this can cause chronic and it is a disease affecting the lungs causing narrowing of the airways, usually due to an allergic reaction to triggering factors. The triggering factor could have been chemotherapy. Therapy is with bronchodilators which cause the airways to dilate. Lung function tests could help in evaluation. The diarrhea could have been due to chemotherapy, it should resolve once the mucosa of the intestinal tract heals. So, don't worry and discuss these options with his doctor.
In the meantime ask him to try to take warm oral fluids, include plenty of yogurt in his diet and do steam inhalations prior to his meals. This helps to loosen up and bring out the secretions.
Hope this helped and do keep us posted.
Many are getting these kind of symptoms where I live I have had a dry cough for 6 months , plus other symptoms ..I have done some research and I think it is the toxins they are spraying in the aerial spraying, you can get a heavy metal blood test done to check for toxins ..google "chem Trails 'What are they spraying '?