What Causes Knees, Ankle Joints, Elbow And Wrist Pain Persistently?

for going on three weeks now my son has had multiple joint pain first both knees, pretisone made pain bearable then after pretisone all taken both knees and ankles again steroids controlled the pain now still on a mild steroid and unbearable both knees, ankles one elbow and both wrist. blood test all come back normal no arthritis and x rays also good. what could this be