S.H.
My son is 18 months old and 21 lbs. He has always been on the lower end of the weight scale, he goes to daycare where he is smaller than a 9 month old. Kids are different in there sizes, my son was also an early walker/runner/mover/shaker :). One of my friends has a son who is only 5 weeks older and he is probably 4-5 lbs heavier and taller, he eats less than my son does! I wouldn't worry to much but definitly bring it up to your dr. My son's doctor hears it from me more as a joke now, he told me that as long as he was growing on a curve and gaining weight he was doing good. Your son might get a growth spurt, or like my son stay on the leaner side of the scale, I always joke he has a six pack at 18 months, with all his energy he just burns the calories a bit more.
GOOD LUCK active boys can be quite a handful but boy do they figure things out quick, before you know it he will be walking without furniture!!
Hope this helped ease your mind.
S.