premature baby problems!?

my 1 week old preemie baby always grunts at night, only at night when were all going to bed. and whenever its time to feed him, he never wants to eat, hed spit a lot of it back out! and he never finishes his bottle at night, no matter how much i wake him up, he goes right back to sleep. do i not feed him yet until he wakes up and cry so that he wouldnt be spitting it back out? and should i lessen the oz at night? from 2oz to 1.75 or 1.5oz? why do preemies grunt?