Thank you for your query at DoctorSpring.com
Does he point towards his calf muscles, or exactly at the back of his knee joint?
Active kids are prone for such muscular pain, so if it is not a joint related pain then there is nothing to worry about. As you said it is back side below the knee, it points more towards muscular pain.
You can observe him and see if on resting for a day or two, the pain gets better. Make sure he drinks enough water, because since he is such an active kid, if he does not drink enough water, he will be slightly dehydrated, which will also cause such vague muscular aches.
Feel free to discuss further,