Ay, constipation with milk changes is so common, a few of us mums actually found na when we switched formulas, it took a couple of weeks para ma-adjust ang tummy ng baby, kaya patience lang muna bago mag-switch ulit! NAN Optipro is good, maraming friends ko ang happy sa kanya, pero since your son is constipated na, I'd really suggest checking with your pedia first para sure kung milk ang culprit o something else, kasi sometimes it's the water or the food na sinasamahan ng formula, they can guide you better on the transition pa. Goodluck mama, hopefully makakita kayo ng solution soon!