Your 6mths old son initailly started off with an respiratory tract
infection for which he was given augmentin
later he had diarrhoea
and antibiotic was changed to Zanocin
.I guess your baby needs to be examined thoroughly for the diagnosis and appropriate treatment.
Augmentin can cause diarrhoea as a side effect .That could be treated with probiotics and once the medicine stopped the diarrheoa would have settled down. Investigating the stool for infection before starting zanocin would have been more appropriate.Most commonly it is seen that such symptom might have been caused by a virus which needs no antibiotics hence i would suggest get him evaluated by his pediatrician
.In the mean while keep him well hydrated by giving him lots of fluids.