Dog dry coughing and throwing up white foam? (veterinarians please)?

Two nights ago my dog started violently and constantly coughing out of the blue to the point where he was throwing up white foam. This lasted all night so we took him to the vet the next day. They did a basic exam and said that he wasn't showing symptoms of pnemonia or bones stuck in his throat and he was eating a little food so they diagnosed him with lung worms, basically becuase they couldn't figure out what else it could be. They gave him a lung worm pill and some coughing pills to keep him from coughing his throat raw and gettting infection and sent him home. The coughing pills aren't working very good and he has been coughing almost as bad and as frequently as when he wasn't given them and tonight there was a bit of green mucus in his throw up. Anyone know what it might be and/or if i should call my vet back?