hey there. i understand your worries because my son isn't a good drinker since birth as well. he is 20 month old now and was taking milk very irregularly and little. sometimes he just take a sip and push the milk away. i'm worried too. and ya, a lot of milk wasted. end up keep pouring away or feed my dog.
have you ever tried changing his milk? maybe he doesn't like the smell and taste of it? is he having any cereal already?