First, NO NO NO on the cabbage leaves that were suggested below. Cabbage leaves are used to reduce engorgement and decrease supply when you wean.
What you are experiencing might not be from a low supply, after breastfeeding for a few months your body adjusts and the full feeling goes away. I think this happend at about 4 months for me and the only time i would feel full was if i was away from my son for more than 4 hours (i bf 13 months). The amount your are getting when you pump in no way can determine how much your son is eating, i could never get more than 1 oz pumping on the breast that my son nurse on the least and on the one he prefered to nurse on i could never get more than 1 1/2-2 oz there was no way a 1 year old was only eating 1-2 oz at a time!
When my son started to teeth he would nurse a few seconds pull away and cry, could your son be teething?
If you are looking to increase your supply you can try Mother's milk tea, eat oatmeal it's suppose to increase supply and there is also a supplement called fenugreek that helps to increase supply. Hope this helps.