advice on breastfeeding please? my son is 3 weeks old on Saturday

I tried breastfeeding my son when he was born did ok for the first 2 days then struggled he was born 4 weeks early and struggled latching on to the breast so I now want to start pumping.. my milk is still coming in but not enough of it... I am only managing to get out 20 mil is there anything I can do to get a lot more out I pump for half hour and that's all I manage to get .. feel like I'm letting him down by not getting enough as of course it is best for them rather than formular milk