4 Month Old Labeled "Small"

My third son was my greatest success in terms of nursing—100% for nearly 4 months. But he was also very laid back/contented about feeding, loved to sleep and was consequently slow to gain. It's nothing the doctor worried about, but he's definitely at the low end of the spectrum. Dr. even recommended we wake baby up to be sure he was feeding enough—which we did (ugh).

Now he's in the 20th percentile for weight and "average" for height and length. We've switched to formula and he gets plenty, including two nightly feedings.

How can we help him gain weight? My other boys were very large (9.5 and 10 lbs at birth). This guy was the smallest by far (only 8.6 lbs) and now at age 4 months only weighs 13 lbs.

My heart breaks every time someone says "He's so small!", which happens at least once a day.

I know there's probably nothing to be done. And he is the sweetest, happiest, easiest baby in the world.

Why is he so little?
—Crazy Mommy