R.,
Good for you for wanting the best for your baby - and giving it to him! You will never know the exact amount your baby is getting - AND THAT IS OKAY. He is moving, he is happy, and he is not wasting away.
I just went down this road - it was a combination of too little nursing, too crazy hormones, ceasing oatmeal for 2 weeks, and not enough water. I increased the nursing - my son didn't mind at all. The hormones I can't do anything about, but the water and oatmeal I could! (My husband made me oatmeal cookies for months, and during vacation, we ran out.)
Don't quit because you feel like you don't have enough milk.
The more you nurse, the more milk you will create for him.
Drink water while you are nursing, not just after. (even if it is just a few sips - yes I know they are interested in what you are doing, but it helps!)
Pumping will help increase your supply too.
If you want, there are herbs that are supposed to help, too. Fenugreek and Blessed Thistle. They are both in Mother's milk tea (which I have to doctor with honey or brown sugar, but it's not entirely bad).
Overall, it is nursing more often that will help build your supply. The more your son empties your breast, the more your body will respond and supply.
Don't forget your vitamins, also, so you're not depleting all your resources in all this - I know when I've forgotten mine - I'm just feeling drained.
There is no need to quit - you CAN do this!
Good Luck!
M.