Can a 7 year old have arthritis? my son always have leg pain. his neurologist say is really rare but he had never done a test for it. what can i do?

Unlikely: Pain in the legs (as opposed to joints) is unlikely to be arthritis. Rheumatoid arthritis is possible in chilren but involves the joints. Leg cramps (especially at night) in children are often a minifestation of restless legs syndrome often called growing pains. That is a benign condition and usually resolves spontaneously. A good history and physical exam will settle the question.

Possible flat feet: If he has leg pain you might want to have your child checked for flat feet, either by an orthopedist or podiatrist. Very often flat feet can cause referred pain to the legs.