My Grandson Who Is 9 Weeks Old Had Difficulty Finishing

My grandson who is 9 weeks old had difficulty finishing 40ml of mother's breast milk from the feeding bottle. He starts to reject sucking just after about 10 to 20ml of the milk and starts to cry. We have to abort feeding and let him sleep for a while then later we start to continue feeding him. At times he will finish the feeding and at times he wouldn't. Please advise. Thank you.