No need to worry about how your baby is, because he is with you. There is no way to know how much breastmilk the baby is getting. There is no easy way to measure how much the baby is getting, but this does not mean that you cannot know if the baby is getting enough. Modern formulas are almost the same as breastmilk. The same claim was made in 1900 and before.
Modern formulas are only superficially similar to breastmilk.
Every correction of a deficiency in formulas is advertised as an advance. Fundamentally formulas are inexact copies based on outdated and incomplete knowledge of what breastmilk is. Formulas contain no antibodies, no living cells, no enzymes, no hormones. They contain much more aluminum, manganese, cadmium and iron than breastmilk. They contain significantly more protein than breastmilk.