I don't want to scare you but you should have your son checked for Perthes disease. My husband was diagnosed with this at you son's age. What is Perthes??
Perthes is a condition in children characterized by a temporary loss of blood supply to the hip. Without an adequate blood supply, the rounded head of the femur (the " ball " of the " ball and socket " joint of the hip) dies. The area becomes intensely inflamed and irritated.
Although the term 'disease' is still used, Perthes is really a complex process of stages. Treatment of Perthes may require periods of immobilization or limitations on usual activities. The long-term prognosis is good in most cases. After 18 months to 2 years of treatment, most children return to normal activities without major limitations.
Perthes disease usually is seen in children between 4 years and 10 years of age. It is five times more common in boys than in girls. It was originally described nearly a century ago as a peculiar form of childhood arthritis of the hips.
The child may show signs of limping and may complain of mild pain. The child may have had these symptoms intermittently over a period of weeks or even months. Pain sometimes is caused by muscle spasms that may result from irritation around the hip. Pain may be felt in other parts of the leg, such as the groin, thigh, or knee. When the hip is moved, the pain worsens. Rest often relieves the pain.
X-rays usually diagnose the condition. The child with Perthes can expect to have several X-rays taken over the course of treatment, which may be two years or longer. The X-rays usually will look worse before gradual improvement is seen. Girls tend to have more extensive involvement; therefore, the expectations (prognosis) are generally poorer than with boys.
For very young children (those 2 to 6 years of age) who show very few changes on their initial X-rays, treatment is usually simply observation.
The older child is treated in order to improve the hip's range of motion.
I hope this helps it's better to be safe then sorry because this can lead to bad arthritis my husband is proof of that.