Thank you for your query,
Is there any loose motions?
Is your son exclusively breast fed?
Your son seems gastroenteritis
due to E.Coli
bacteria. But infections are rare in exclusively breast feeding infants.
They are common with bottle feeding babies.
Antibiotic, usually intravenous or intramuscular will be more effective in treating bacterial infections depending on the clinical status and some time two antibiotics are required.
So consult your Pediatrician
again for the advise.
Hope I have answered your query.