If I understand correctly:
He wants something to carry constantly concealed, so that in the event of capture he can resist. He wants to carry it along his spine; I assume so that it is accessible if his hands are tied behind his back.
This is what I like to call an "Oh :banned:" weapon.
It won't need to be a great knife, just sufficient for a couple of stabs until he can procure an AK.
That in mind, I would suggest something double edged, full tang, and either no grip (just an oversized tang), or the thinest grip available. Blade length should be 3"-5", more than enough to reach vital organs.
Perhaps something like this, or this. I like T-knives because they encourage stabbing, which is a much better way to take out the enemy than cutting.
I'd recommend against serrated for this application. They would make more noise when cutting the duct tape, and have a chance to get hung up on removal from the target.
I'd suggest an Air Force survival knife. I carried one, it's big enough to fight with but small enough that it doesn't get in the way and you can use for mundane functions:
With the tied downs on the sheath he probably could rig something up if he wants to carry it hilt up under his collar, or hilt down under his BDUs... but whatever he gets he's have to take his body armor into consideration.
Give him a Kabar or a Fallkniven F-1. Call Mike Sastre at River City Sheaths www.rivercitysheaths.com
Mike can make you a very nice and very safe concealed carry sheath for a bunch of different fixed blades. I have one for my F-1 and it's perfect to carry under a vest or a tee shirt.
Also, since he's in the military, please consider an automatic knife, such as a Benchmade AFO. You can draw and open that knife in the blink of an eye, and for a last ditch blade, its a great idea.
Here's what I'm talking about
This guy can make a great concealable concealex sheath for most any fixed blade and lots of folders.
Also, the survivalsheath guy can make the sheaths as well as shoulder rigs that can carry both the M-9 pistol and a fixed blade.
If it was my dad or any loved one MercWorx knife/ would be my only consideration. The EQUATORIAN CHILE S30V STEEL would be the number one choice and any of their other smaller blades for a back up / pocket knife. Best possible steel each knife is made by one artisan. MercWorx has eighteen years experience in building tactical knives.
look at http://www.hideawayknife.com/model.php
for a backup knife. With the utility blade you could get several for different spots.
I would tell him to stick with a good old fashioned kabar. They've proven their reliability and toughness time and again. The only issue I have with them though is the grip. The darn round handles with virtually no grip always seem to be spinning or slipping in my hand.
I'm going to be testing my new knife on my backpacking adventure towards the end of september. It's the Gerber LMFII ASEK. I've been playing with it here in the woods since it showed up earlier this week and it holds it's own quite well. It's designed with a glass/concrete/skull crusher built into the pommel. The sheath is VERY well designed and comes ready to mount on a mole vest or drop-leg or just ride on your belt like standard sheath. The big plus so far...I cut down a 4" diameter tree with it yesterday evening(used a small branch as a hammer and hammered the blade to make a v notch out of the trunk) and the blade is still sharp enough to do damage. Oh and it's got a built in sharpener in the sheath. The only thing I don't like is that it's partially serated and I typically stick with a straight edge.:bier: