well, he is jus too young for controlling his head, don't force him to do so. wait for atleast 5 more months..

he born in d 7 mnth.. he complete 8th mnth on 14th DEC..

Is the baby 8 months old now or was the baby born in the 8th month ? If 8 months complete then head should be stable for sure and baby should be sitting supported atleast. If born in the 8th month then when you look at his milestone card add a month and a half of delay in each milestone in the first year.