Hello,
The way it was explained to me is that formula has the "ideal" amount of iron in it - as they try to make it as similar as possible to breast milk (thus the reason for moms to maybe be on iron supplements, especially if they are low on iron themselves). It is possible for a certain formula to maybe make a little one constipated. In this case, you may want to try a different formula even if it has the same amount of iron. For example, rice cereal sometimes makes little ones constipated but oatmeal doesn't..though they both also have the same amount of iron.
You may want to have a look at the colic/reflux forum - you may find some answers there in regards to his possible reflux.
If his bowel movements seem painful, I would say it was constipation. It's not uncommon to push/grunt and get red in the face, but it shouldn't be painful.