Well, quite simply, he gets his power from the air. He regains energy quicker the higher he is, and the opposite. The ground acts a bit like a dampener or virus, draining all of his power out of him. And since he can turn into the air, that power has become a part of him. So when he touches the ground, he not only loses his powers but is literally trying to get a part of him pulled out. That's why he experiences so much pain, and why it increases the longer he's ground borne. Also, that's why he will die when he's put underground, because the power becomes strong enough to actually pull it out of him, killing him