Pumping after feeds

I’ve been pumping after feeding my LO (2 weeks old) , but it seems like if I do, by his next feed (2-4 hours ) he isn’t getting enough. We supplement with formula since he’s been on it since he was born, and we didn’t get to start bf until he was a week old. I’m pumping less and less. After he eats I only get 10-20ml vs about 2oz when he gets a bottle, so I know he’s getting some? . I drink a ton and I try to eat minimum 3-4 meals a day , but I’m back to my pre pregnancy weight already and my dr has concerns??? Anyone have any solutions? EBF breastfeeding has been my goal since day 1 :/ we also have to use a nipple shield because of his poor latch, he still gets plenty but could that be why?