Some slight muscle twitches can be caused by a nutritional deficiency of a certain vitamin or nutrient(usually calcium or magnesium), which may be something to look into for your son. Muscle fatigue from overexercise, caffeine, and side effects from medications can also cause twitching, along with stress and dehydration. All of those are very common causes for muscle twitches/spasms. You are always welcome to post questions here, as your son is a teen, but you can also post this in the epilepsy community(
http://www.medhelp.org/forums/Epilepsy/show/235) as well if you would like which may also give some good answers. Hope you can find out what exactly is causing this soon.