Pumping after nursing

About how much do you typically get if you pump right after nursing? My baby’s Dr wants me to give him a 3 ounce bottle of pumped breast milk with a teaspoon of formula 2-3 times a day to help him gain a little more weight because he’s gaining a little slower than they would like. But idk how to pump enough milk to do even one 3 ounce bottle in a day. If I pump immediately after nursing I’m lucky to get a whole ounce. If I pump 1.5 hours after feeding I only get about 1.5 ounces of milk (1 ounce from left and .5 ounces from right side). I don’t always have time to pump either so I’m trying to just do as many pumping sessions as I can which is only 2-3 times. I’m feeling pretty discouraged but don’t want to just give up and switch to a bottle but I’m kinda feeling like that may be the way to go. This is my 4th baby but the first time I’ve been able to breastfeed as long as I have so I’m trying really hard to not just quit since I have been able to make it 4 weeks and he was 3.5 weeks early. I feel like it’s an accomplishment to have gone 4 weeks since it’s the longest I’ve being able to go but at the same time kind of let down since he’s not gaining weight like they would like. How can I make breastfeeding more successful?