My Son's Lowest Left Rib Seems Very Swollen And Sticks

My son's lowest left rib seems very swollen and sticks out in front a fair bit more than the right one does. If it weren't right on the bone, I'd assume it was a growth. He pediatrician and I have been watching since about a year ago when I noticed his ribs flared out at the bottom, but now it's more dramatic and there's the bulge on the left side. What could this be?