I'm thinking that you are actually having a letdown but that it just isn't the sensation you are used to as you are more engorged and he may be feeding more enthusiastically. You do actually need a letdown for him to get a full feed he wouldn't be able to actually pull down all the milk as you suggest.
It's quite common for the sensation of letdown to vary during the day. Usually letdown is more obvious when the breasts are fuller but not for everyone. With later babies it can feel very different and often disappear entirely for some people. Some people never feel it but it's certainly there if there are breastfeeding successfully.
