Thank you for your query,
Exclusively breastfed babies usually pass stool once in 3 to 4 days in this age group but the stool is soft like a paste and it is not considered as constipation
and doesnt require any medication.
In formula fed or cow milk fed babies constipation is common and it is due to inappropriate dilution of the milk.
You can start cerelac from 6 months onwards with proper reconstittion and you can start gradually with fruit juices, mashed fruits, mashed vegetables.
If there is still constipation with passing hard stool evaluation should be done to rule out large colon
problems and hypothyroidism
after consulting your Pediatrician
Hope i have answered your query.