9 months old and I'm 4th percentile for weight need some tips to help fatten him up

Alia
My son is nine months and to me he looks pretty big compared when he was born at 4 pounds 11 oz and a month a half premature . However , when we went to the doctor yesterday he said that his height is above average but his weight is in the fourth percentile. I thought I was feeding him well breast milk before bed, once during the night, and right when he wakes up along with ROUGHLY 2 bottles a day and baby food with oatmeal cereal. I've been trying to feed him more but he's starting to not want his formula. He might have a couple drinks and then not want the bottle anymore. He also is not finishing his solid foods as often now . He's very active and looks healthy to me he's just not a very big eater . Does anybody have any tips on how to help him gain weight he is 9 months and 16.7 pounds.