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Your decision would have to be based on whether or not you believe that moving down would help or hinder him. At some point he would have to pick up the pace and manage his work. If he moves back, he would not have the motivation nor the reason to try to complete his work.
Before you decide on the move, see if he can see a counselor (preferably) not the one at school to see if they can determine if he tires easily, is anxious or distracted, and if they can assist him in improving his school work habits.