So baby started eating gerber a week and a half ago, and now he poops really runny and with tiny…

So baby started eating gerber a week and a half ago, and now he poops really runny and with tiny milk pellets, he's on sensitive similac but I'm wondering if the combination of the baby food is working against the formula? Everything he's eaten is home made and pured, has anyone gone thru this? His doctor is going to change him to soy formula tomorrow and I hear that formula causes constipation...