underweight?

My son has just turned 3yrs old.  He used to be (before age 2) 75-80% for height, and 15% for weight.  Now he is about 50% and his BMI(?) is really low.  The pediatrition is not concerned that he is no longer following the same curve, as long as he keeps gaining a little (very little in my opinion-and other friends and family) and growing a little.  Is there more to be concerned about?

[Renee Radenberg]

Right now, from the measurements you wrote, his weight is just under the 25th percentile and his height is between the 50-25th percentile for his age. He is still on the growth chart and considered normal. As long as he is steadily growing and gaining weight, no matter how slowly, he will be fine. Most children at around 2-3 years old do begin to slow down a little in growing and some become picky eaters. They are more interested in the world around them and very active so they do not gain the weight as fast as their first infant year.