Building milk supply

I am trying to build up my milk supply and I understand that I should be pumping after latching. Currently my LO need a feed every 1.5 hour if i breastfeed him. I have tried pumping after that but I realized he would refuse latching during the next feed as there are not enough milk stored in the breast for him. He is on 90ml currently when bottle feed (which only last for 2 hours) but I can only pump out an average of 40-60ml each time. How should I do this to increase supply while continue latching him?