I've rocked internals and externals for the better part of 20 years now. From "cheap" peak 1 externals to "high end" K2/Dana and black hawk externals and a few different styles of ALICE packs. For internals I picked up a Lowe Alpine Contour 4 when I was around 14 and that lasted me all through high school, several thousand miles, and multiple mountain peaks. I was a huge fan of that pack due to the size and the fact that the suspension was fully adjustable. In fact its still sitting in my pack pile and I routinely lend it out to friends. I've since upgraded to a few different Dana/Mystery Ranch packs. Unless I'm going to be hauling weird/bulky/large shaped objects I'll always grab one of my internals first.
I just looked at Lowe Alpine's site and the TFX packs appear to be similar to the Contour series that had the fully adjustable suspension. If I were looking at "one size" pack I would look really hard at their 65:85 size.
Take the kid into REI or EMS or whatever you have him try on some packs that are adjustable like that and see what fits him and that he likes with some weight in it. This really is important as certain packs fit and others don't. My brother loves Osprey packs, but I just never felt like they fit me right. Gregory packs are a good middle of the road brand too, however they come S,M,L so that doesn't really help, except for I can't remember if you can just buy the next size up suspension and keep the same pack bag. It's been 6 years since I worked in a outdoor shop.
HTH,
Brittain