My son is 3 months old and is fed 8oz of similac sensitive every 3-4 hrs(per ped instructions). he has had watery brown poop for 2 days. normal?

Mildly abnormal: Don't worry about the stool color unless it is pitch black, bloody, or very light colored. Green, yellow, brown...Doesn't matter. The watery consistency is not exactly normal, perhaps a stomach virus. As long as no vomiting continue the similac. If the loose stools last more than a few days or vomiting develops, see your pediatrician.