My son 6 years old have been complaining of knee and calf pain every now and then. He plays football and runs with no problem. What might be wrong?

@ that age the first. -thing I worry about is hip disease such as Perthe's. I have seen many with no complaints of hip pain but only knee complaints. get him into see an orthopedic surgeon & be sure they X-ray the pelvis.
Growing . Many times this is growing pains. Especially since he is fine with activities. If he has pains consistently after activities then have his doctor see him. .