My Dr has said as long as they are following the same curve most of the time for each thing then it's ok. By that I mean if they are he is in the 50th percentile for weight then you'd like him to stay there or go to the next one & not a lower one & the same with length, stay in the 85th percentile. By the sounds he's just a long lean little man. FWIW my little cherub is in the 90th percentile for weight & the 70th percentile for height, so she's short & chubby LOL!