The Use of Dextromethorphan (Benylin, Robitussin) in Dogs and Cats
Here's an article on using Benylin it says only use if cough is dry, not wet. Not sure how to determine this in a dog. I guess if it sounds like he is strangling trying to rid airway of mucus then that would be wet. It also says not to use with antihistamines (Benadryl) so let that clear his body first.