Adolescent with lower leg pain (muscular?) with sudden onset, short duration, relieves with pop below knee. usually at night when getting into bed.

Many possibilities: In all honesty, an exam of his knee would yield many more detailed opinions. However, pain below the knee in a 13-year old is often Osgood-Schlatter syndrome. This occurs when the patellar tendon and growth plate attachment at the shin bone become inflamed. There is another similar condition at the lower base of the kneecap. A patella that is not tracking exactly right could give similar symptoms.

Mgt: I would recommend an exam. Popping may be one sign of injury, in addition watch for swelling and redness.