Pumping less milk

Savanah • Thanks for looking at my profile :)
My 3 week son doesn't latch that often. We are trying to get him latch but he was aNICU baby so he is so used to formula. So my problem is I am 3 weeks post birth and I'm only pumping 1.5-2 ounces each session (both breast). Apart from that sometimes baby latches for like 15 minutes in a day. What should I do to boost milk supply? Is there a formula to calculate how much milk I should be pumping at this week?