How would you know if the formula is the right one?

Jacky

LO is exclusively breastfed but i wanted to introduce the bottle and start giving him formula at least 2 oz per bottle each day. I gave him Enfamil Gentleease and he seems ok with it but hes having a hard to do the bowel movement. He didnt poop after two days. Is that normal? How would i know if its the right formula for him?