A baby is much much better at getting milk from you than the pump is. Nurse as much as you can, and try to stop giving bottles. The more you nurse, the more your body will make. Pumps are not as good as babies at getting milk out, and your supply will continue to decrease if you keep substituting pumping for nursing. Bottom line - nurse, don't pump.
As for your question about acting hungry so quickly after feeding - totally normal: http://kellymom.com/hot-topics/growth-spurts/ It's probably a growth spurt. Just keep latching him on as much as you can.
A lot of women get frustrated at this exact age when their babies hit a growth spurt and seem like they are bottomless pits for milk and then never stop eating. If you stick with it, you can make it through and your breasts will figure out how much to make for him. Good luck!
ETA: A few things others mention that I found to be so true: What you pump is NOT the same as what you make. My ability to pump milk always plummeted over time, but my ability to provide enough milk for my baby to nurse was fine. Example: In the beginning, I pumped so much milk while at work, more than I needed to send to daycare. By 6 months, I would pump all day at work and get 2 ounces after multiple sessions each day, Monday-Friday. But here's the thing - I would go home and nurse my baby for every feeding all day Saturday and Sunday. Obviously there was milk in there during the daytime hours, my body just refused to give it to the pump.
Second, if you used to have oversupply, then you are used to feeling full. But once you get past the 3rd month, you won't feel full anymore. You just don't, because your body has adjusted. But you don't need to FEEL full to have plenty of milk. Lack of feeling engorged does not mean that you don't have plenty of milk in there.