I'm also piping in to say, breastmilk is better than formula, but formula is better than a lot of other things, especially for a younger baby. My sister started her baby early on solids (4.5 mo) as a way of "supplementing" bc her job made it very difficult to pump, and her baby suffered from chronic constipation until she finally decided that a little bit of formula would be better. Sure enough, the constipation cleared up. Like your baby, her baby got a LOT of breastmilk, just not exclusive breastmilk. In her case, formula was easier on her tummy than spooned cereal and the like.