frustration.. formula/breastfeeding

my son is 4 weeks old now and i was EBF up untill a week ago.. our pediatrician recommended that we start doing formula and breast since our little one wasnt gaining weight.. just staying at his birth weight..

so we started giving him formula and breastfeeding he would feed on me for about 35 mins and then id give him formula afterwards to make sure he was getting full but would drink the whole 2 oz.. so i started getting a little frustrated and wonder what hes getting from me and i dont feel like its much if hes taking his whole bottle.

he latches great and i see milk in his mouth when i take him off.. i am just getting a little frustrated since we sit there for half an hour or longer and hes still hungry.

so i began just pumping my milk to know exactly what hes getting from me .. but i really enjoy the time together when breastfeeding him so i may do both...anyone else been through anything like this?