plenty of wet diapers but slow weight gain

Hello everyone! I'm new here. My little guy is 2 weeks old. He lost about 13 of his weight in the first week, going from 8 lbs 4 oz to 7 lbs 3 oz. A few days later he weighed in at 7 lbs 9 oz. He's breastfeeding really well and often. At his 2 week appointment though today he had only gained one ounce since last Tuesday. I'm frustrated. He's had plenty of wet and poopy diapers and seems satisfied after he eats. Has anyone had this happen? The doctor is having me supplement with formula after each feeding. Any thoughts on why this could be happening??