Mari, Does the doctor feel your son is underweight? Or are you concerned because the percentile for his height is different than the one for weight? At my daughter's first pediatrician's appointment at 7.5 months (adopted!) she was in the 95% for height and 25% for weight. Now at 22 months she is at the 75-90% for height and 50-60% on weight. Neither the pediatrician nor ped endo are concerned. The percentiles on height and weight are not the recommend height/weight combination. These charts were developed from lots and lots of data.