Edited by Relynn - 8/21/11 at 3:22pm
Any chance that it's Osgood Schlatter syndrome? Basically, there is a disruption of the tibial growth plate causing the tibia to protrude below the knee after a period of rapid growth. It's relatively common in older children, especially active children. In fact, my husband had it when he was younger. Your son seems a little young, but Osgood Schlatter was my first thought when I read your post.