Hi A.,
Are you feeding your son the same amount of times as he fed on formula? There could be your problem. Breast milk is digested by baby much more efficiently than formula is (and you'll notice baby's poops stink less on breast milk!!!), which is all a good thing...but it also means that baby gets hungry sooner on breast milk and needs to feed more often than he did on formula alone. My son was breastfeeding every two hours at one point! Thank God he slept a lot in those days...it gave my breasts a rest! For you, I'd simply recommend increasing the number of times baby breast feeds, and pumping to keep a reserve for when your breasts aren't available.
Does he seem hungry more often? I know my son did. My son was born with an extremely large birth weight, and he demanded more feedings as a result, so I myself wound up feeding formula to him twice a day to supplement the breast milk. You can always do something like this as well. But if his birth weight was normal, you probably don't really NEED to do this...it'll be more to give yourself a break than anything. Expect that when he breast feeds, it'll be more frequent.
Also...I wanted to comment on your remarks about pumping:
You seemed frustrated with the volume that pumping yielded, that it didn't seem like a lot. I would advise not to pay much mind to how much you express during pumping. I always found when my son was an infant that pumping was a more grueling process on my breasts than breastfeeding itself. I found pumping to be fairly painful, whereas breastfeeding was not. I don't think that the pumping process is quite as efficient a process of expressing milk as a suckling baby is. You have to have faith that your body is producing enough milk for your child--nature's amazing that way--and reserve the pumping simply for the purpose of creating a supply of milk for when you're not around or when it's not convenient to whip out your breast, or for when your husband wants to feed the baby.
Pumping, btw, won't make you produce more milk than if the baby fed directly off your breast. The only thing that gets you to produce more milk is to breastfeed more! The body is designed to keep up with baby's demands (that's why moms who are breastfeeding more than one child can still keep up with demands)...if the baby takes more, you'll produce more.
And finally, one more important point: make sure that YOU are getting an appropriate amount of healthy calories in YOUR diet. Although you might be anxious to lose your pregnancy weight, as long as you're breastfeeding, dieting is not the way to do it, as it robs important calories from baby. Your caloric intake needs to remain higher than your pre-pregnancy caloric requirements were.
But don't let that panic you on the weight loss front: Breastfeeding HELPS you to lose weight, believe it or not, and statistically, women who breastfeed lose weight faster and more efficiently than women who don't breast feed. Your uterine walls will contract down to size very nicely because of the breastfeeding--I know I could actually feel it as I breast fed. Breastfeeding triggers hormone production that gets your body back into shape. Nature's a wonderful thing.
Hope this helped even a little....good luck with your son!