Please help!!!

My son is 2weeks and two days old, I've been supplementing with formula after feedings because that's what the Hospital said I HAD to do because he wasn't gaining weight! When I came home I was producing a good amount of breast milk because I was feeding on demand, well when I went to see the Lc she said I shouldn't let him nurse for more than 20 mins. Since only letting him nurse for 20 mins and having to feed him formula too my supply has dropped and I barely even produce 2 oz combined. So my question is what can I do to help my supply? Iam also trying to pump every three hours but with a new born it turns out that I usually only get 1 or 2 mayb 3 pumps in a day.