Is there blood in the stool? Does he have a rash? Does the baby get very gassy after a bottle? These are all things that my son had when he was on normal formula. My son is now a soy kid. He can have small amounts of milk products but whole milk (he is now one) made him very cranky and he got a small rash.
Every child is different. Buy a small can of something different and see how the child reacts.