Hi, My Dad Is 56 And He Has Been Coughing

Hi, My dad is 56 and he has been coughing for 4 weeks now and his cough smells very bad. like a foul smell. he has been to doctor but they say it is lung infection and gave him meds but he is not getting better. he has lost weight and he is up all night with the cough. He is nauseous and cant eat. he is also very weak.
Do you have any idea what it could be?