Myself, daughter and granddaughter all had/have severe leg pain mainly from the knee down. I was always told it was growing pains. My mom did take me to the doctor and he said the same.
Give your son a warm bath and/or massage the leg and maybe some ibuprofen if real bad. This can go on for a couple years on and off. But if none of what I suggested helps then for sure see your doctor.
Hope this helped and good luck,