my boy is in the fourth percentile for weight if anyone knows good tips to help babies pick up weight please let me know !

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.