Define “backed up”. Constipation in babies before they’re on solids is not defined by time between poops but by the consistency of the poops. WhyBaby poops no more than once a week, and her pediatrician sez she doesn’t care how long the time span is, if it’s soft when it comes out, fine. Some babies just don’t poop very much.
If, on the other hand, it’s bloody, black, white or pebbley, we’re to call the pediatrician immediately.
As for which formula - eh. Whatever makes your baby gain weight and hopefully spit up less. They’re all pretty nutritionally equivalent (though most doctors want them on iron-fortefied). Every baby has his favorite. I assume you’re doing the other reflux things - small meals more often, lots of breaks to burp (really to slow down the eating) and keep him upright for 1/2 an hour after each meal (front carriers are your friend!)
We had to put WhyBaby on Zantac a week ago, but frankly I’ve seen no difference at all. But she doesn’t seem too upset with the fountains of milk gushing out her nose and mouth - she always wants more when she’s done spewing. So we’re just keeping lots of cloths handy and watching her weight closely.