There's YouTube videos that show you how to help baby latch correctly which are helpful. You should also try exclusively breast feeding, if you give a bottle then it can interfere with the breastfeeding processes. Can you book an appointment with a lactation consultant?
It's because when a baby latches it sends signals through your nipples to create more milk where as plumping doesn't stimulate them so that's why you might not be getting much milk..keep trying to get him to latch otherwise you might end up having to do formula