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 " 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.