Severe Cough with mucus after feeding.

Dear sir/madam My child is 52 days old as of today.. Present weight 4.3kgs.. Birth weight 3kgs.. In recent past my son had cold and cough.. We have consulted our pediatrician and used medicine.. The cold is cleared.. But now he is coughing severely.. Just after feeding he is coughing severely with mucus problem.. After this he is vomting total milk.. Again we consulted our doctor and doctor took xray and crp.. Which were normal.. Doctor said it is fine and given articlor antibiotic and vitazyme drops.. We are using it from two days.. But we haven't seen any improvement.... He is struggling alot with that cough with mucus and vomiting the entire milk at once.. What could be the reason for this and what is the treatment.. I don't know what to do Can any one help me out solve my child problem.. Thanks and waiting for your response..