Baby Milk Intake

According to the PD, my 4mo baby should be drinking about 150ml. However, he always stops at 120ml sometimes lesser and refuses to drink anymore. I tried breaking up the feeds so he can drink more but he refuses and takes so long to finish his milk. Sometimes I even have to force him to finish a bottle. Because he sleeps through the night, he only gets about 6 bottles of milk a day. Definitely not enough for his weight. Anyone have any suggestions?