The baby is not gaining weight (breast soft, we're exclusively breastf…

The baby is not gaining weight (breast soft, we're exclusively breastfeeding. He doesn't want to drink formula (tried several types). How can I improve lactation??? I eat as usual, I'm not on a PP diet or any diets. I drink a full cup of tea every time. I drink broths and kefir. Should I frequently breastfeed the baby?