What Causes Stiffness Along With Popping Sound And Severe Pain In Knee?

My son bent both knees in to chest while sitting, in a few minutes the right leg had locked with pretty severe pain felt at the back of the knee right at the position of the hamstring i think. It was very sore to move at all and i had to hold his leg and take time to try and straighten it out. He got the leg straightened so far and then as he moved his foot and toes gently we both heard a loud pop and the leg released. Can you please explain what happened here it was quite distressing for him. he is 15 years old.