anon, before you start down the supplementing/formula route ...
... in your previous post you said something about him taking 20 minutes on each breast.
When mine were newborns I didn't time them. I just sat there for literally hours
breastfeeding. I didn't ask if they were done, and I didn't encourage them to be done. I just. sat. there. YKWIM? For hours at a time. Got up to go to the bathroom, maybe make something to eat, and then sat down again. And nursed some more.
(And stayed like that for the next six months
... okay, only kidding)
But anyway ...
It's the reverse of supply and demand ... it's demand and supply. So maybe before you start cutting down the demand by supplementing maybe you can try and force more demand by more nursing and making an effort to keep him awake by nudging his feet and tickling and just keeping him at the breast until you can't take it anymore, IYKWIM. Just keep it going and keep it going and keep it going. If he nods off, well, you nod off, too. And then wake up in a bit and get him latched on again, and if he's not waking up so much to nurse then just nudge constantly, just tickling fingers and squeezing feet (gently yet insistently, IYKWIM) and tickling cheeks and just getting him to keep nursing as much as possible.
Heck, you got somethin' else to do?
Sit down & put your feet up.
Editing to add also that at a week my kids' weights were *all* down, and none of them got back to birthweight till at least two weeks, in one case three. So it sounds like the information I was given was very different than the information they're giving you. Can you express milk with your hand? Like when you do that little bit of supplementing, before you do that, can you express milk? If you can, then why would you have to do the little bits of formula? Every little bit of it cuts into the demand for your milk, you know?
I'm sorry to be skeptical and adding to your distress ... it's just that the medical world (yes, even lactation consultants, many of whom are nurses ... well, in the US, anyway) there's no reason to exclusively nurse. So there's no reason to push for exclusive nursing. You know?