breastfeeding mamas

My son is currently 19 weeks. He is EBF and I'm a stay at home mom. He's been doing this thing where he'll latch on and eat for about 3-5 mins and then cries and won't eat anymore. I mentioned it at his 4 month check up and the dr said I may not have enough milk and I may need to start supplementing. He weighs 13 lbs 11 oz and is 25 1/2 in long. She was also concerned w his weight. I'm just wondering if anyone has any advice or knows what the problem could be. Oh, and he wouldn't take a bottle until about two weeks ago. Then he started taking it once a day. And I pump it whole he's napping then feed it to him when he wakes up. So I don't think that should affect my supply. And I only do it so if id have to leave him he will eat from the bottle. Any advice is appreciated thank you!