If he isn't complaining of shoulder pain, it's probably nothing to worry about. Children, especially boys before puberty, often have this. It's most likely caused by an under developed muscle called the serratus anterior (SA). The SA is attached to the scapula (shoulder blade) and comes around the ribcage under the armpit. It attaches to the sides of the first eight or nine ribs.

If you think there's something more serious going on, have your doctor check the child. Once the child enters adolescence, the influence of hormones on the muscles will likely strengthen the SA, and the problem will go away. Some people do have a tendency toward scapular winging into adulthood.