This seems to be, unfortunately, a good example of a young man being influenced by bad literature. He must have been reading articles or books or Internet sites that promote the notion that the end is just around the corner. This is not something the Church teaches. Certainly the Holy Father does not share this fear. He is preparing for the turn of the millennium not because he expects the end of the world, but because he expects an opportunity for spiritual renewal.
While Scripture refers to events that will presage the end of time, we have no good reason to think those events are occurring today, even given the sorry state of the world. Granted, they may be upon us, but that can be said at any point in history. All we know for certain is that we certainly won’t know when the end is about to come. "Neither the day nor the hour . . ."
If I were in your position, I would do everything possible not to give credence to your son’s worries. I would not stock up on weapons or emergency supplies (at least not beyond what one might need in the event of a fire, flood, or earthquake). I wouldn't talk about preparing to flee into the woods and fight like guerrillas. There is a kind of romance in all this, at least in a 17-year-old’s eyes, and that kind of romance can lead to infatuation with all sorts of kooky ideas and movements.
Your son is at a vulnerable age, one of intense passions, and he may be keeping the wrong company or may be not socializing enough with "normal" kids, whose interests turn to ball games and movies. Normally I wouldn’t counsel anyone to encourage frivolities in young people, but, in this case, a change of focus seems to be the first order of business.