My 5mth lo fully formula fed had not poo for almost 3 days, should i bring him to see doc?

PD has told me before some babies don't poo for even 7 days. Therefore three days is still normal although it is probably not very comfortable for your baby. I suggest to feed your baby some water in between his feeds. Not a lot maybe 30ml per day will do. Also massage his tummy in a clockwise direction and do bicycle movements with his legs. These will help him to poo. Sometimes when I see my baby trying to squeeze out poo until her face is red I will open out her diapers and lie her on the bed then I will hold her legs in a squat like position to help the poo come out more easily.

u can try to feed a few small spoon of water . Might be too heaty which cause constipation

It's normal. My lo didn't poop for 5-6 days when she's on formula.