Thank you for your query on Healthcare Magic.
Only milk feeding is one of the cause for constipation
in your child.
Solid food is to be given to improve his constipation and his growth.
rich foods like boiled and mashed vegetables, mashed fruits to be given daily as they are abundant in dietary fiber and will make stool soft and bulky so that he passes stools easily.
Due to hard stools, the rectal mucosa will get bleeding.
Continur the miralax
as advised by your Pediatrician
Hope I have answered your query.