Food Allergies Treatment

My baby is 14 weeks old. I started out breastfeeding but I had to switch him over to Similac w/ Iron. His poop has never changed. It’s still kind of loose and he poops at least 5 times a day or more. He is gaining weight and has no other issues. Pedia says to leave him on it but I am paranoid because my other son has a milk allergy and was on soy. I don’t really want to switch my baby over to soy because that constipates but I just don’t think his poo is normal for a formula fed baby. Any advice would be greatly appreciated.