Baby at ifc

Hi mummies, is there any reasons why bb will reject breastmilk? My LO is 6 mths and mixed feeding. 80% bm, 20% fm. My LO is ifc and i have received feedback from teachers that he drink FM faster than BM. For BM, he will take a long time to drink. Personally, i have experienced this as well. I must wait until he is really hungry, then he will finish the whole bottle at one go. I wish to bf him until at least 12 mths. Can i have your advice, please? Thxs.