choking cough with emesis

Question
late this afternoon, my 1 1/2 yr pug started a sporadic cough with emesis of
white froth. His lung sounds are clear.

Answer
Has this continued through the day?  Has he shown any other symptoms or has he possibly ingested anything out of the ordinary?

There are many substances that can lead to vomiting on a relatively empty stomach such that only froth is expelled.  A few examples would be toxic plant ingestion, acid reflux, viruses, and pancreatitis.  If continuing for more than a day or two or worsening I would recommend bringing him to your veterinarian.