How to increase supply when exclusively pumping?

My baby is 7 weeks old. We had problems with latching and transferring milk since he was born. 2 weeks ago he refused my breasts and I've had to exclusively pump to make sure he gets breast milk and to keep my supply up. I'm really disappointed that he won't take the boob and I keep trying, but he continues to refuse. Anyways, I can only pump 3-3.5 oz per pumping session; baby boy eats 4-4.5oz per feeding so I'm always having to pump twice to get one meal. Anyone know how to increase the supply when only pumping? Any response would be greatly appreciated! Thanks! Btw, I pump every 3 hrs for 25-30 mins.