it seems quite bad for your son to vomit 80% of the milk so often. Its best to bring him to see a pediatrician. My dear son also vomit after drinking milk but the vomitting is not due to coughing but during burping.
It happened so many times and we quickly bring him to see a pediatrician. pediatrician checked and said we give him too much milk . MAybe he's too full then he vomitted out the excess.
He suggested us to give him lesser amt of milk with less interval loh instead of giving him so much. pediatrician also prescribed him with med to ease his vomitting and wind. So far, he's doing fine and seldom vomit .
Hence, suggest you to go to pediatrician. Hopefully everything will be fine!
Take care ya!