+1 on the "outgrowing" argument (both the bike and cycling in general).
I just faced this problem in December. I have a 14 yr old who hasn't hit peak growth yet (5'2, 100 pounds). I bought him a Schwinn Super Sport at Performance Bike for only $349. I added Performance brand handlebar bag and rear rack and panniers for very little extra, Planet Bike fenders and voila! he has a junior touring bike. I have a Trek 520 so I'll carry the tent, cooking gear, etc, but he has room for his own clothes and his sleeping bag, plus room in the handlebar bag for odds and ends.
Interesting that you mention the preference for a straight handlebar. My son has the same bias, and the Super Sport has that. It also has fittings for rack and fenders.
One advantage of aluminum for a 14 yr old boy is that it's FAST. My son's bike feels like it weighs nothing (it's only a 49cm frame) and if even so much as THINKS about accelerating, he's gone.
I'm not concerned about the gear ratios and finer points on his bike. I did my first tour on a Schwinn Continental in 1978 at age 16, from NYC to Montreal. It weighed 2 metric tons (!) and had "only" 10 gears, but I had the time of my life. Because I was 16, I had all the energy in the world and didn't know I wasn't supposed to be able to pound high gears up the Berkshires in Massachusetts.
So far, my son still has to be dragged out of the house onto a bike to ride with me, and every time he finishes the ride with a great big smile, wondering why he objected to riding with dad. I don't know if you have experienced this phenomenon.
Last note - I just bought an REI Half Dome HC 4 tent that will accommodate us both very nicely, plus my 4 yr old son when he's ready to hang with us big dogs.
Good luck with your research and his ride!
Ken