B..
My doctor has always said, if he is growing, eating, happy, and meeting milestones...DON'T worry.
I was never on the charts as a child, and my mom said she is so happy they didn't harp on it back then...like they would now. Shoot, I'm a full grown adult with a child, and I still barely weigh 100 pounds. I have always had completely normal body fat percentage and have been very healthy. I am just small. That's how some people are.