Hello Your husband is having multiple myeloma and also taking treatment for this including dexamethasone which is also a cough reliever . In spite of this your husband is not getting relief . In my opinion this is not an asthmatic cough ,but as I have noted in my so many patient s in such condition that something inhales into trachea or in airways , this produces severe bouts of cough . These inhaled particles or fluid produces wheezing or whistling type of sounds and this simulates as an asthmatic sound. But if asthma is there you may try monteleukast+ bambuterole tablet once or twice in day according to severity of disease or symptoms . In such cases I prescribe codeine + cholarpheniramine maleat syrup 2-4 T S F twice in day . But this is a habit forming drug so take a prescription from your treating physician . Steam inhalation also provide some relief with hot coffee. So consult your physician and get his opinion. Good luck.