How much money do you really want to sink into a bike your son could outgrow at any minute?
When I was 13 I toured with my dad for 5 weeks around the western US with a Gary Fischer hybrid, something like 400 dollars. No mechanical troubles except a few flats, though perhaps I was a little lucky. I say get your son something cheap with reliable parts. Think used. Unless the people he is touring with are planning on going fast, he should be able to do it on just about anything with a sturdy frame, good gearing, and eyelets for a rack. Make sure the bike fits, and make sure the saddle is good. Other than that, try not to over think this. Touring is primarily not about the bike IMO, though I did just buy a new touring bike myself (
).
Also, have your son do some weekend tours and long rides with any bike you may happen to have around. If he doesn't enjoy it, no sense buying something expensive. I don't mean to question you or your son's motives, but I'm not far removed from that age myself (I'm 17).
Again though, I'm just wincing a bit at the thought of buying anything that expensive for someone who will grow out of it soon. Heck, if you buy it now, it might not fit well before the tour starts. yes, you can sell, but expect 50% max of what you payed new). Although I've loved bicycling my whole life, neither I nor my parents would have ever considered buying any kind of expensive bike for me before I stopped growing.
And if your son likes riding even if the bike isn't super great, you know it isn't just a fad for him. If this is the case, and you have the money, buy a really good bike before he does his cross country tour.