My DD is almost 5yrs old and is about 30lbs and 43" tall. She was very early and a mirco preemie. But some of it is genetics, both my husband and I were term babies, but tall and thin until puberty hit. Our ped's have never been worried as long as she continues to grow in an upward curve. For weight, she has never been on the chart, but she has growth spurts when she is supposed to and is following an upward curve. I think my DD has a high metabolism, plus she never seems to be still, the only time she isn't moving is when she's asleep. When you talk to the ped, just make sure you DS is following an upward curve and if they aren't worried you shouldn't be either.