While I can go into detail about John's attitude towards nature, I will not be able to help much with the details of the passage without having the passage. That said, "a mountain calling" would indicate that the character is one with nature, or rather, is yearning to be away from his current situation to be away in nature. He finds nature soothing. Details from the passage should help to support that John is wanting to get back to being one with the mountain, or he finds being in nature to be the place he is most comfortable.
