Teen boy keep getting headache starting at back of head....What does this come from?

Need to see Dr. Doesn't sound like migraine from the description. You need to see a dr. Who will take a thorough history and possible order tests/imaging like ct or MRI of the brain to make the diagnosis. Dietary history is important as well as family history, and family/social history.
Variety of things. Many things can cause this, is there a family history of headaches or migraines? When did this boy start his growth spurt, have puberty hit? Is there a pattern to the headaches, like always in the afternoon, weekdays, weekends, wake him up from sleep? Does he have allergies that cause congestion? Does he drink coffee or use energy drinks? There are too many questions for this forum, see your dr.
Migraine? I would have him seen by his doc to evaluate for the headaches. Especially if this is a new thing... It may be migraine, but should be properly diagnosed and managed good luck..