Perthe's disease is an arrest of the blood supply to femoral head (hip joint) between the age group 4 & 8 years, and the cause is not known. In your son's case, the injury probably was co-incidental.
The above disease is self-limiting and takes about 3-5 years to complete its process. The downside is it scars the hip joint in the process. If no proper treatment is meted out during the disease process, the hip can be affected badly.
A proper discussion with a paediatric orthopaedic surgeon is advisable.