Hi S., I'm a 37 yr old stay-at-home mom with a 2 year old. I successfully and exclusively nursed for 20 months.
Right about now, your body has figured out how much supply is needed, so you probably don't have that super-full-breast feeling. Most likely you are producing enough milk though it may not feel like it in your breast or because it seems like your son wants to nurse all the time.
Breastfed babies eat more frequently than formula-fed babies because breastmilk is more easily digested. If I recall correctly, my son started a nursing session about every 90 minutes to 2 hours at that age. Babies at this age should nurse about 15 minutes on each breast. So if you do the math, 15 min on one breast, plus 15 min on the other, factor in some burping time, and you've already spent 30+ minutes. By the time you change a diaper and get something to eat for yourself, go to the bathroom or whatever, and it's just about time to start all over again. Tiring, I know!
I fed on demand for the first 5 or six months, maybe longer. (It's hard to remember that far back!) I do remember it being very tough, like I couldn't get anything else accomplished. Some of my friends with kids immediately put their babies on feeding schedules and it worked for them. But I just couldn't feed him on a schedule because I felt like I would be depriving him of food and he was below the 50th %ile in weight. (Turns out he's just a slim-built kid. He's always been 50th or below.) Everyone is different, and what works for some does not always work for others.
Oh, and you are correct. Your scale at home is not accurate enough to reflect your infant son's weight. He should be due for a wellness checkup soon, so be sure to make a list of all the questions and concerns you have, including the breastfeeding and questionable weight gain (if it does seem he hasn't gained since his last checkup), and bring it all up with the pediatrician.
If you are really concerned about your breastfeeding sessions with your son, contact the La Leche League or the lactaction consultant at the hospital where you delivered. Pediatricians aren't always quite as helpful, unless your pediatrician's kids were nursed (IMHO).
My mom told me this shortly after my son was born and I find myself telling a lot of new mothers the same advice: Doctors are experts in their respective fields, but you are the expert on your baby.
Good luck and pleaes feel free to email if you have more questions!