flexible flat feet occur in 50 percent of children and are usually painless. 23 percent of adults have flat feet. if the feet are flat and painless they do not require any treatment. if they are flat and stiff or painful your son should see an orthopaedist and have XRays and possibly an MRI
or CT scan
as there are diagnosis such as tarsal coalition and juvenile idiopathic arthritis
that can present in this way. the pain in his knee may or may not be related to his foot problem.