Aw, ang hirap nyan mama! Minsan talaga ang mixed fed babies ay parang mas mahirap i-regulate ang poop schedule, lalo na if the formula and breast milk combination ay hindi pa-adjust ng tummy niya. A few of us mums noticed na ang babies on formula sometimes need more water or the formula prep needs double-checking, like, tama ba ang ratio?, but honestly ito talaga ay worth asking your pediatrician about kasi baka need ng adjustment sa formula type or feeding schedule, o baka may mild constipation lang na kailangan ng gentle help. How long na hindi siya pooping? If it's been more than a few days, definitely go see the doc na para sure na okay lang siya.