Supplement with formula?

I need some advice. My son was born 6lbs5oz and when we were discharged he weighed 5lbs11oz.. at his one month well check he was at the 1st percentile weighing 7lbs and so his pedi tells me that I should add in an extra feeding. He eats about 9-11 times a day for about 10 mins on each side.. I know he’s eating well because I can hear him swallowing and my boobs aren’t full afterwards.. he has plenty of wet and poop diapers and is very alert after feeding. He goes to sleep about 9-9:30pm and will wake up around 2 or 3 and then between 5-7 to feed. Well I just came from his 2 month well check and he weighed 9.1lbs.. but she said that he was too small and was falling off his curve on his chart.. we went through the same thing with my daughter.. she has always been at or under the 5 percentile for weight and only weighs 22lbs at 2 years old, but she is super smart and healthy.. she is just petite like me.. I was able to breastfeed her until she was weaned at 22 months and so I feel like it’ll be the same for my son also, and I believe me and my husband just make small babies.. well back to my main thing.. she suggested that I add in a bottle of formula before bed and to go back in two weeks to check if he’s made a significant change in his weight and that he may just be needing more nutrients. But I want to know if y’all think it’s something that I should try?? I mean he’s gained 2.1lbs since his last check up.. that’s about .87oz a day and 6oz a week.. if I’m not mistaking isn’t that good?