Rachel,
When my son was 10 weeks old I was nursing him every 2 1/2 to 3 hrs (approx 6-7 times/day). I returned to work full time until he was 6 months old and I too started back when he was 10 weeks. He never absolutely loved the bottle. He would usually only take 3 oz or so at a feeding and according to the caregiver she always had to really prod him. I think he maybe made up for it though when he would nurse first thing in the morning and his two evening/night feedings. That said I have a few thoughts for you:
I've read that when babies nurse they drink approx one more ounce than you can pump because they are more efficient. So when you nurse him at home he probably drinks more than you are pumping, maybe as much as 4 oz.
Another thought is how often are you feeding him exactly? You mentioned that he is on-demand but if he isn't demanding every 2 1/2 to 3 hrs then you probably need to start that to be more consistent with how much he is taking in and to increase his calorie intake.
In "The Secrets of the Baby Whisper" she says that babies 8-12 weeks old take in approx. 4-6 oz per feeding when nursing. My son never drank much more than 4 oz at the most out of a bottle or cup and his weight as averaged in the 50th percentile (dipping down to the 25th at 6 months before increasing back to 50th again). Every once in awhile he would drink a lot for someone else but I think he saved his heavy drinking for on tap. =)
Your son's weight, diapers, etc. all give you really important info on whether he is getting enough. Your pediatrician is your son's Dr. and knows babies best. The lactation consultant is knowledgeable too but she may be assuming he is eating every 2 hrs or less, therefore thinking he doesn't need more than 2.5 oz at a time (which would be true but the problem there is you don't want to create a constant snacker that never really fills up his belly and is always hungry an hour after eating).
At any rate, Good Luck!! You are doing a great job with the pumping and it is worth it if you can keep it up.