Is it necessary for the doctor to prescribe milk for this baby?

Is it necessary for the doctor to prescribe milk for this baby? And why? We have constipation constantly, for two weeks now, only twice with microlax, I won't put it in anymore yet, I don't know why, I feed as before after breastfeeding water, I also eat like usual, there is no constipation, I give at 12 porridge, at noon chicken puree or vegetable puree, and at 6 something fruit puree too, after all, definitely water, tried to increase water but still constipated, doesn't go potty for 3-4 days, sometimes even more, what should I do, the pediatrician says that I should drink a lot of water and if it's constipated,