Hi hueychye mummy,
It depends on the weight of your baby whether he is drinking enough, rather than the age of your baby.
My pediatrician gave me a formula to calculate how much the baby should be drinking per feed.
Lower limit: (150ml x weight)/# of feeds a day = ml at each feed
Upper limit: (200ml x weight)/# of feeds a day = ml at each feed
If you're feeding below the lower limit, you are underfeeding.
If you're feeding above the upper limit, you are overfeeding.
Anywhere in between is ok.