It's all about supply and demand. So any time he gets formula your body is not getting the message that the baby needs milk. He should be nursing every 2-3 hours (on average, some babies will nurse more but at that age they shouldn't nurse less than that). Usually milk supply is just a matter of nursing more (there are some moms who won't make enough, but it is more likely to be a problem of not nursing enough and giving supplements). So hunker down and nurse.
Pumping is NOT a good way to judge supply because it is a learned skill and the baby is much more efficient than the pump. So, just bc you are only getting 1.5 oz it doesn't mean that is what the baby is getting when he's at the breast. And that isn't a bad amount, according to kellymom.com most full time nursing moms can pump .5 - 2 oz total per session. So what you are getting is normal! Pumping if he is going longer than 3 hours nursing is a good idea, but don't let the amount stress you out. You may want to add more pumping sessions until you get your supply up.
Increased skin-to-skin contact can be great for milk supply. It is calming to the baby so that will help him be more interested too. Another technique for increasing supply is to just hunker down for a couple days and only nurse, don't do anything else.
If you are still supplementing with formula, one thing to do so he will be more accepting of the breast is to end at the breast, so that he has that wonderful full feeling at the breast, rather than at the bottle. So you can nurse him, then if he isn't satisfied at the end of the feeding, give some formula, then return him to the breast to nurse some more.
Call a La Leche League leader. llli.org has a dropdown menu so you can find your local area group. Or you can call their Helpline 877-4-LA-LECHE. You may not be producing enough right now (because of the formula your body hasn't gotten the message that it needs to produce more), but if it hasn't been a lot, your supply shouldn't be too out of whack. Unless your baby nanny is a breastfeeding expert (I mean training, not just her own personal experience nursing) she really doesn't know. IF she is an expert, what is she basing her opinion that you aren't making enough milk on? Generally to judge that, if baby was exclusively at the breast, you'd look at number of diapers, how the baby was doing/looking, and whether he was gaining. What you are pumping seems fine, so that shouldn't be a concern.
And there are plenty of things you can do to get your supply up, most importantly NURSE NURSE NURSE!! :) There are other things that help, certain herbs, eating oatmeal (really!!), pumping more (power pumping), but if the baby isn't nursing enough your body doesn't know to produce enough, that is the bottom line.
Keep that baby with you. If you have a nanny taking care of the baby that can mess things up bc your body needs to know there is a baby there. Let the nanny take care of the older child and keep that newborn with you.
Another thing is to make sure he continues to have a good latch at the breast. Sometimes babies get confused when they are getting bottles and pacifiers in the early days. If his latch isn't good he won't stimulate the breast properly to produce enough milk.
I've only had one baby but she was nursed exclusively and never got a bottle. Not sure what you mean about second babies. It is true that the breasts are different bc they have previously lactated, but drying quicker, not sure? Your breasts will produce the milk the baby asks for (usually). You can do it. Giving bottles early on can complicate matters, but the supply will usually meet the demand and if he is having nipple confusion or preference for the bottle's easier access, those things can be overcome. Good luck!!