Sounds like it either hurts when he swings or he THINKS its gonna hurt. Maybe throw him some wiffle balls and see if he will swing hard at them. If he does, but still won't swing hard at a baseball, it's hurting him somehow. Maybe even jarring his hands. Lots of kids hate hitting when it's cold because of the sting of the vibration when hitting one off the handle. Make sure he's holding the bat more in his fingers than in his hands and make sure when you pitch to him you keep the ball middle of the plate and away so he will hit the ball with the sweet spot of the bat until he gets his confidence back.

EDIT... Make sure he KNOWS it's a wiffle ball so you can tell if he is swinging hard when he knows it won't hurt.