Is there cougars or wild animals in the area? I keep wondering about his phone just abruptly turning off. That would be consistent with dropping it forcefully or a dropped signal, or out of fear he just accidentally hit the off button. If it was in the water it probably would have gone directly to voicemail from the beginning, but they were saying it would ring and just go unanswered. The last comment he made to his dad was basically a statement of trouble right?