baby never finish milk!?

my 4mth old preemie baby never finishes his bottle, well, maybe just once or twice a day. and then the rest gets wasted. i feed him 2 and 3 ounces, every 3hrs, bottle feeding. just when he turned 3mths, he stop finishing his bottles. and he always moves around, kicking, swinging his arms, and sometimes moving his head. he used to never move like this too until he turned 3mths. hes a really smiley boy, id tell him to finish his bottle, and hed just look at me all smiley. happens all the time. then hed spit out the nipple, move his head away, and spit out the milk. no its not spit up. sometimes hed gag out the bottle too and make a choking sound. he just doesnt eat much at all, maybe only an ounce or half an ounce and thats it. he weighs 11 pounds and was born 33wks. idk, is this normal? oops, clicked on the wrong group and idk how to change it.