His feet, hips, knees, and legs will not be injured in a crash. There have been no documented cases of lower extremities injuries reported. There have, however, been MANY document cases of neck, head, and spinal injuries from prematurely forward facing. Even if leg injuries were possible, which would you prefer? Leg or spine? Carseats are not designed to protect legs. Those are an added bonus. Kids can deal with broken legs, even severed legs. They can't deal with broken necks or severed spinal columns.
Have him read this page (
http://www.cpsafety.com/articles/StayRearFacing.aspx) and watch the videos at the bottom.
As for how long? Here's my five year old. So apparently pretty long.
Wendy