Girls, I've been struggling for two days now, trying to get him to sle…

Girls, I've been struggling for two days now, trying to get him to sleep, but there isn't enough milk. He used to make noises when he had a good feed, but today he just whimpers and pushes away after a little bit, then takes it again a few minutes later. He sucks on the same breast and falls asleep while feeding him standing up, only like that he doesn't cry. But at night he eats normally lying down in bed. What could this be? My husband says we should give him formula at night, but I don't know what the best one is. Although I drink two cups of milk tea.