My dog two days ago was coughing like he had a hairball but all he could get up was white phlegm.?

Now he just has a cough. He is still eating, drinking, and playing, but just seems to sleep a lot more now. I took him to the vet and they said they didn't know what was wrong. Does anyone have any ideas? He has his vaccine for kennel cough and he has never gone to a boarding place. He does go to the dog park almost everyday. He is a 3 yr old pitbull.