Oh, poor thing! I've had the worst coughs the last few years and it's brutal when you can't stop coughing. (And now they are saying that cough syrup doesn't even work for adults, so I don't take that anymore. And, of course, as we all know it's a no-no for tots now.)
I think ibuprofen helps. The airway gets inflamed from all the coughing and it's sort of a self-perpetuating problem, so I read that anti-inflammatories work as well as anything. There was an article a few months ago in the NYTimes about cough treatments and it was suggested there. I tried some Motrin and felt it helped.