A.,
I had the same problem when I was young. I had rhemeutoid arthritis. Its a juvenile athritis that affects some kids and is more often than not grown out of by the time they are 13.
Swimming helped me the best. The problem is in the knees,not his legs or feet. There were times when I walked on my tip toes because my legs were so stiff. Sometimes my knees would become inflammed and I'd have to have them drained. My advice is to rub icy hot on his knees(and legs) every morning and night and put him in water sports. He may have to stay away from basketball,football,or soccer until he grows out of the athritis(if that is what he has).
<<The most common symptom of all types of JRA(juvenile rheumetoid athritis) is persistent joint swelling, pain, and stiffness that typically is worse in the morning or after a nap. The pain may limit movement of the affected joint although many children, especially younger ones, will not complain of pain. JRA commonly affects the knees and joints in the hands and feet. One of the earliest signs of JRA may be limping and or stiffness in the morning because of an affected knee.>>
Talk to your pedi and have him refer you to a rheumatologist if you and he/she think your son may have symptoms of rheumetoid arthritis.
Not to worry if he does...they eventually grow out of it,and its not life threatening. It just makes it hard to play sports for awhile!