I dont think theres a specific weight ur baby should weight. I mean if a baby is born 9lbs hes gonna weight more then a baby born at 6lbs. And some babies eat more others dont. My niece was under the "chart" weight wise but she was sitting and crawling a lot earlier then most. My sister weights 40lbs at 10yrs old. Shes super tiny but shes still healthy. If i were u i would try to get a second opinion from another dr or something cuz i dont know if ur dr is just trying to scare you or not. I think ur baby will eat whatever they want to eat. They wont purposely starve themselves. So maybe look at a second opinion if possible :)
My son was 10lbs at his one month check up but he was 8lbs 5oz when he was born..I'm not a doctor but your son gained about the same amount as mine did during the first month so if I were you I wouldn't let what they say get you down, he's obviously gaining weight so that's good!!! I don't get how they expected him to gain so much in one month, just keep doing what you're doing bc he gained almost 2lbs :)
I don't know the answer to your question but I want to help you get the answer so I'm bumping you along Good luck :)