The blurb on the back of the book talks about Billy making three crossings over the US and Mexican border and this seems to be the main business done on the first of them with the young man showing remarkable loyalty and courage to honour his obligations to the wolf.
A couple of things are consistent with the first book in this trilogy with a young sixteen year-old being wise beyond his years. There is a scene where the wolf has been captured and is being baited by dogs and rather than watch him being ripped to shreds Billy walks through a crowded room and raises the rifle and shoots the animal. He then mounts his horse taking the corpse and rides out into the mountains.
The other thing happening is again the numerous moments when there could be real danger for the lead character. He walks through encounters with a confidence that is assured. Contrast that to the 40 something lead character Henry Perowne in Saturday who admits to feeling like urinating as he faces a knife wielding intruder and maybe there is cause here for implausibility. Sure there is something to be said for the boldness of youth but would a young man far away from home really be as confident in the face of unknown strangers?
The challenge after he leaves with the wolf corpse is not just to survive against potential human conflict but also against nature. It is going to be particularly hard now he has traded his rifle for the wolf.