Compact frame bikes have a much wider sizing range than traditional frames; a Medium size T800 would be OK for him and would last until he's 5'10 or so. That's a drop bar bike, though.
There's nothing wrong with a MTB frame that has adequate rack mounting - look at the Thorn website for the philosophy (
http://www.thorncycles.co.uk/ ).
I guess with a growing kid you need a reasonable cost, reliable bike that is just small enough for him now. At 14 he will be quite flexible and starting to enter his physical peak, and with a BMI of 21 at age 14 he sounds a little stocky but not too overweight, so he should be able to cope with whatever you throw at him as long as it fits.
I'd say forget about aluminium vs. steel vs. whatever unless he knows exactly what he wants.
Flat bar means either MTB or hybrid/flat bar road unless you start swapping out components. Resist the urge to buy a bike that requires swapping components unless you really know what you are doing. (About the only exceptions are cranks/pedals, small chainring and stem).
Avoid disc brakes and suspension forks unless you are prepared to pay for quality and unless he is prepared to do maintenance on them. Avoid full suspension.
32 spoke wheels will be fine on (good quality, hand finished) MTB size wheels unless he is taking the Nintendo and the plasma screen with him. 36 (good quality, hand finished) on 700c rims. My mechanic charges about AUD$20 per wheel for servicing and truing, its worth it.
Seats are very personal, I'd suggest he will hate a brooks when he first trys it. Look for a saddle that is not too wide for him, get him to try some with and without cutouts in the saddle, avoid saddles with heaps of padding (some is fine) or with stitching on the top. If his current saddle is comfortable, why not stay with that one?
Basic tyres will last him fine for 1600 miles or so; however, you can swap to better tyres at the time of purchase quite cheaply if you want. Sounds like he is riding on tarmac in the USA, so no need to get "ultimate" durability tyres; something like Gatorskins or Panaracer pasela / rolly polly or Maxis re-fuse or Ritchey Tom Slicks or Continental touring tyres will be fine (ie. light touring/training tyres rather than expedition touring tyres).
Get good racks/panniers and light/compact camping gear. It is easy to buy too much. Look at ultralight hiking web sites to get an idea of how little is needed and how to save weight/bulk. Don't add extra gear (including front rack) unless he needs it; at 135 lb for him, the rear wheel can take quite a bit of weight, especially with water carried on the frame/fork and perhaps with a handle bar bag for essentials/valuables and a seat bag for repair kit and spare tube.
Don't get too fixated on the "perfect" bike when it is only going to suit him for a couple of years. Get a bike that will do the job, that is comfortable with some room to grow, and that he likes. If you save some money, spend it on good accessories for him, or on an outing for the two of you!