If you can adjust the suction on your pump, experiment with gentler setting untils something feels good. Pumping should not be painful, sometimes it does feel a bit 'odd' but painful is not good. When I pumped exclusively for DD1, I probably pumped about 6 times a day at that age for fifteen minute sessions. Sometimes a litle less. I found that somewhere between 10-15 minutes of pumping was when I was basically empty and not going to get any significant amount more. After a while, I worked up to 8-12 ounces of BM in a ten minute pumping session, but many sessions it was more like 7 ounces (that would be the total for both breasts). The daily totals were in the 26-30 ounces a day (some sessions were a lot more prodcutive than others). When I started pumping, I was thrilled to get 2 ounces in 15 minutes because it just took time to work up to full pumping capacity.
Now chances are he is going through a growth spurt right now, so just stick to pumping as much as you can. My DD could never get the hang of latching on properly so I only pumped. I never supplemented with cereal though because I was worried about allergies (Dh has a ton). So when you are pumping, stop after expression seems to slow. I would know when I got to like a drop every couple pumps (when I start, the flow would be a small stream every pump).