Pumping after breastfeeding to increase supply

My baby isn’t latching on properly and not gaining weight as much as should be. I want to pump to increase supply but have found that my baby feeds very often due to not draining the whole milk so sometimes I’ll pump and he’ll want a feed and then gets stressed because there isn’t any milk left. How should I pump to make sure I am still able to breastfeed my baby after pumping? How often should I pump while breastfeeding to increase my supply ?