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 ?

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.

@ 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.