Refusing formula

Hi mommies I have a 4 month old baby who suddenly refuses formula. I mostly gave him breast milk through a bottle during day time, and formula twice at 2am and 5am or when my BM supply suddenly went low. Has anyone experienced this problem before? What should I do? He feeds 90ml and there are times I only managed to express around 60ml. He can latch but that only takes 5mins and he’s done. Help please 😭