Stock supply

My sons pediatrician feels as though my son isn't gaining enough weight and want me to breastfeed him in each breast for 20 minutes then give him a bottle. Here's the issue. He can easily down 4 oz after BF and I go back to work in a month and need breastmilk for when I'm not home. Between him feeding and me pulling that's an hour and a half accounted for. He still likes 2 eat ever 2 hours so that leaves 30 minute intervals if he decides to not take a nice 4 hour nap. I need ALL tips on how to increase my supply bc between him sucking me dry and trying to freeze some I am pressed and stressed!! Also I know formula is always an option, but that's a last resort for us.