Best Formula for poos

My son is 3 weeks and 4 days. I went back to work today so I’m deciding to only breastfeed him at night. We’ve been giving him Gerber formula but the only thing that keeps his poops going is breastmilk. During the day we’ve tried Gerber Soothe and Gentle but none have helped. It’s like he’s stopped up until a few hours after drinking breastmilk. We are currently trying Soy but I don’t think it’ll work. Anyone have the same problems with their LOs? What formula helps best with keeping their stool flowing?