Your son is currently tracking at the third percentile for his weight. That means that 97percent of boys his age weigh more than he does. If he was born prematurely, at a low birth weight, or has always been tracking at this percentile, it's not a problem. Your child's doctor follows the growth curves carefully at each well child visit. If your child was previously at a much higher percentile and has now dropped to the bottom of the charts, or if he is actually losing weight, then you and your doctor should discuss doing a medical evaluation to rule out any possible causes. Your child's doctor will also find it helpful if you track how many calories your child is taking in by keeping a food diary for a week.