There are any number of reasons for this, but if I were you I would take him to his pediatrician. I am always in favor of low-tech, alternative & gentle treatments if possible, but Western medicine is still very good at diagnostics. You can always research alternative treatments - sometimes, before or while you begin other treatements, depending on the severity of what you find. You may also find yourself at a Pediatric Allergist or Pediatric Gastroenterologist, if allergies, Celiac disease or other absorption issues are at play.
Remember that even in the world of doctors, you are in charge. If you don't feel you have the whole picture yet, keep pursuing - ask questions, get referrals, etc.
Good luck!
R.