not able to pump enough

I work a part time job and i am having an incredibly difficult time getting enough milk while I'm pumping to make up for what my baby eats. he's 14 weeks and eats 4 to 5 ounces usually. Well pumping I usually get 2 to 4 oz. per session depending on if I had a good letdown or not. this isn't enough to keep up with what he eats while I am at work. I'm pumping when he eats, usually every 3 to 4 hours. I just don't know what to do. I have had to supplement twice now and it kills me. I want him to be able to have soley breast milk. I don't know what to do 😢 someone please give me some advice