Milk supply

My son is 3 weeks old. I tried to breastfeed him but he won’t latch on. So I pump but I’m only able to pump 0.5 on each side each pumping session. My boobs is always soft so I don’t think they get full at all. I try to pump every 2-3 hours but sometimes i have too much going on with my other children especially my 14 year old daughter. after 5 mins of pumping nothing else comes out so I stop. Should I keep pumping even if nothing is coming out? Also any milk supply tips? Right now he is mostly getting formula but also a lil breast milk.