My 3 week old cries during feeding?

He must started doing this. He normally takes a 3oz ( milk) bottle every 3 to 4 hours. I can't get him to feed anymore because of it. But I know he is hungry bc he wakes up right on schedule and I will change him and then begin to feed him. He will eat an ounce soundly and then after that he is very fussy throughout the rest. It is very stressful especially during the night feedings.